Lemongrass Fish Broth with Potatoes and Seared Fillet — A Light Summer Meal Soup
This bright summer meal soup uses one whole fish two ways: the bones make a clean French-style broth in just 20 minutes, then lemongrass, galangal, and kaffir lime turn it into something aromatic and alive. Waxy potatoes poach right in the broth. A crispy-skinned fillet goes on top at the table. Serves 4 in about 55 minutes.
Recipe by Christophe Rammant
Christophe is a culinary professional with expertise in French and global cuisine. He has developed recipes and cooking techniques that bridge traditional methods with modern home cooking approaches. Christophe focuses on making classic culinary techniques accessible to home cooks through clear instruction and practical applications. He studied at Le Cordon Bleu Paris and has work experience at a two star Michelin restaurant.
Homecook turned pro with a Cordon Bleu Paris graduate
Traditional French fumet technique shows that fish bones give up their best flavor fast — in just 20 minutes over gentle heat. Unlike meat stocks that need hours, fish bones turn bitter and chalky if you cook them too long. Starting with cold water draws impurities slowly to the surface so you can skim them off, leaving a broth that is clean, sweet, and oceanic.
Professional chefs know that a second aromatic infusion is the secret to depth without muddiness. Once the fumet is strained, lemongrass, galangal, and kaffir lime go in for a second round. This keeps the French base clean while layering in a summery, citrusy lift — two distinct flavor worlds in one bowl.
Food science shows that waxy potatoes absorb liquid as they cook. Poaching them directly in the aromatic broth means every bite carries the lemongrass and galangal from the inside out. They stop being neutral filler and become part of the flavor story — which is what turns a refined stock into a proper, satisfying meal.
Ingredients
Recipe yields 4 servings
The Fish
1.4 kg (2 whole fish, approx. 700 g each) Whole white fish (sea bream, whiting, gurnard, or pollack)
Ask your fishmonger to fillet them and keep the bones and head — you get 4 fillets and 2 carcasses for the broth
Amount
Ingredient
Notes
1.4 kg (2 whole fish, approx. 700 g each)
Whole white fish (sea bream, whiting, gurnard, or pollack)
Ask your fishmonger to fillet them and keep the bones and head — you get 4 fillets and 2 carcasses for the broth
For the Fumet (Fish Broth Base)
30 g (2 tablespoons) Unsalted butter
for sweating the aromatics
120 g (about 1 large leek) Leek, white part only, sliced
white part only, sliced
80 g (about 2 medium shallots) Shallots, halved
halved
Amount
Ingredient
Notes
30 g (2 tablespoons)
Unsalted butter
for sweating the aromatics
120 g (about 1 large leek)
Leek, white part only, sliced
white part only, sliced
80 g (about 2 medium shallots)
Shallots, halved
halved
40 g (2 stalks) Lemongrass stalks, bruised and roughly chopped
for the fumet — bruised and roughly chopped
30 g total (stalks and leaves separated) Flat-leaf parsley stalks
stalks go into the fumet; leaves reserved for finishing
4 g (2 leaves) Bay leaves
5 g (1 teaspoon) Black peppercorns
1.4 L (about 6 cups) Cold water
always start cold for a clean fumet
30 g (about 2 tablespoons) Lemon juice, freshly squeezed
from about 1 lemon — added after straining
40 g (2 stalks)
Lemongrass stalks, bruised and roughly chopped
for the fumet — bruised and roughly chopped
30 g total (stalks and leaves separated)
Flat-leaf parsley stalks
stalks go into the fumet; leaves reserved for finishing
4 g (2 leaves)
Bay leaves
—
5 g (1 teaspoon)
Black peppercorns
—
1.4 L (about 6 cups)
Cold water
always start cold for a clean fumet
30 g (about 2 tablespoons)
Lemon juice, freshly squeezed
from about 1 lemon — added after straining
For the Aromatic Potato Broth
600 g (about 4–5 medium potatoes) Waxy potatoes (Charlotte or Ratte), peeled and sliced into 1.5 cm rounds
waxy varieties hold their shape and absorb broth without falling apart
40 g (2 stalks) Lemongrass stalks, finely sliced
the second round of lemongrass — finely sliced for the potato broth
20 g (about a 4 cm piece) Galangal or fresh ginger, peeled and finely sliced
galangal preferred for a more aromatic, citrusy note; ginger works well too
6 g (about 4 leaves) Kaffir lime leaves
torn — adds a floral citrus lift to the broth
Amount
Ingredient
Notes
600 g (about 4–5 medium potatoes)
Waxy potatoes (Charlotte or Ratte), peeled and sliced into 1.5 cm rounds
waxy varieties hold their shape and absorb broth without falling apart
40 g (2 stalks)
Lemongrass stalks, finely sliced
the second round of lemongrass — finely sliced for the potato broth
20 g (about a 4 cm piece)
Galangal or fresh ginger, peeled and finely sliced
galangal preferred for a more aromatic, citrusy note; ginger works well too
6 g (about 4 leaves)
Kaffir lime leaves
torn — adds a floral citrus lift to the broth
For Searing the Fillets
20 g (about 1½ tablespoons) Neutral oil (sunflower or grapeseed)
for searing
30 g (2 tablespoons) Unsalted butter
for basting the fillets during searing
8 g (about 1½ teaspoons) Fleur de sel
for seasoning fillets and finishing bowls
3 g (about ½ teaspoon) White pepper, freshly ground
Amount
Ingredient
Notes
20 g (about 1½ tablespoons)
Neutral oil (sunflower or grapeseed)
for searing
30 g (2 tablespoons)
Unsalted butter
for basting the fillets during searing
8 g (about 1½ teaspoons)
Fleur de sel
for seasoning fillets and finishing bowls
3 g (about ½ teaspoon)
White pepper, freshly ground
—
For Serving (Raw Garnish and Finish)
80 g (about 6–8 radishes) Radishes, thinly sliced into rounds
raw — sliced as thin as possible, added at serving for crunch and a peppery bite
60 g (about ½ small leek) Leek, white and pale green part, finely julienned
raw — matchstick-thin strips, added at serving for freshness and crunch
15 g (about 3 tablespoons) Fresh chives, finely cut
10 g (about 2 tablespoons) Fresh tarragon leaves
leaves only — tarragon adds an anise note that bridges the French and Asian aromatics
10 g (about 2 tablespoons) Fresh Thai basil or regular basil leaves
scatter in at the very end — do not cook
20 g (about 1½ tablespoons) Extra virgin olive oil
a few drops per bowl to finish
Amount
Ingredient
Notes
80 g (about 6–8 radishes)
Radishes, thinly sliced into rounds
raw — sliced as thin as possible, added at serving for crunch and a peppery bite
60 g (about ½ small leek)
Leek, white and pale green part, finely julienned
raw — matchstick-thin strips, added at serving for freshness and crunch
15 g (about 3 tablespoons)
Fresh chives, finely cut
—
10 g (about 2 tablespoons)
Fresh tarragon leaves
leaves only — tarragon adds an anise note that bridges the French and Asian aromatics
10 g (about 2 tablespoons)
Fresh Thai basil or regular basil leaves
scatter in at the very end — do not cook
20 g (about 1½ tablespoons)
Extra virgin olive oil
a few drops per bowl to finish
THE FULL RECIPE
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Instructions
Build the Fumet (Fish Broth Base)
1
Rinse the Bones (Remove the Bitterness)
Hold the fish bones and heads under cold running water for 2 minutes. Use your fingers to pull out any dark blood pockets and remove the gills if still attached. These are the main source of bitterness in a fish broth — taking 2 minutes here protects the whole pot.
2
Sweat the Aromatics (Build the Sweet Base)
Melt 30 g of butter in a wide saucepan over medium heat. Add the sliced leek and halved shallots and cook gently for 3–4 minutes, stirring now and then. You want them soft and translucent — not browned. Keeping the heat gentle lets them release their natural sweetness into the butter without any bitter color.
3
Add the Bones (Wake Up the Flavor)
Add the rinsed fish bones and heads to the pot and stir gently for about 2 minutes. Traditional French fumet technique uses this brief sauté step to coax gelatin and sweet oceanic flavor out of the bones before the water goes in — it makes a noticeable difference in the final broth.
4
Add the Aromatics and Cold Water (Always Cold)
Add 2 of the lemongrass stalks (bruised and roughly chopped), the parsley stalks, bay leaves, and peppercorns. Pour over 1.4 L of cold water — always cold, never hot. Cold water draws impurities slowly to the surface where you can skim them off, giving you a cleaner, clearer broth.
5
Simmer Gently for Exactly 20 Minutes (The Non-Negotiable Rule)
Bring the pot to a very gentle simmer over medium heat — you want a bubble breaking the surface every second or two, not a rolling boil. Skim off any foam that rises in the first few minutes. Once simmering, reduce heat to low and cook uncovered for exactly 20 minutes. Set a timer. Unlike meat stocks that improve over hours, fish bones release bitter, chalky compounds after 20 minutes. Pull it off the heat the moment the timer goes.
6
Strain and Brighten (Don't Press the Bones)
Pour the broth through a fine sieve into a clean saucepan, letting it drain naturally — do not press or push the bones. Pressing forces bitter compounds through the sieve. Add the lemon juice and taste: the broth should be clean, faintly sweet, and smell of the sea. Season lightly with salt.
Build the Aromatic Potato Broth
7
Add the Second Aromatics (The Summery Lift)
Return the strained fumet to a gentle simmer over medium-low heat. Add the remaining 2 lemongrass stalks (finely sliced), the galangal or ginger slices, and the torn kaffir lime leaves. This second infusion is what gives the broth its bright, citrusy, summery character — it layers on top of the clean French base without muddying it.
8
Poach the Potatoes (They Absorb the Broth)
Add the potato rounds to the simmering broth and cook for 12–15 minutes until just tender when pierced with a knife. Food science shows that waxy potatoes absorb the liquid around them as they cook — these potatoes will be carrying lemongrass and galangal flavor all the way through, not just on the surface.
9
Season and Remove the Aromatics (Clean Up the Bowl)
Taste the broth and adjust with salt and white pepper. Fish out and discard the lemongrass pieces, galangal slices, and kaffir lime leaves — they have done their job and you don't want anyone biting into them. Keep the broth at a very gentle simmer while you prepare the fish.
Prepare the Raw Garnish and Sear the Fillets
10
Prep the Raw Garnish (The Ice Water Trick)
Slice the radishes as thinly as possible — a mandoline gives you paper-thin, uniform rounds, but a very sharp knife works too. Finely julienne the leek into matchstick-thin strips. Place both in a bowl of ice-cold water for up to 30 minutes. Professional chefs use this trick to firm up raw garnishes and make them curl slightly — it keeps them crisp and visually striking. Drain and pat dry just before serving.
11
Dry the Fillets (The Most Important Step for Crispy Skin)
Pat the fish fillets completely dry on both sides with kitchen paper. Lay them on a clean plate lined with paper for at least 5 minutes. Any moisture left on the skin will create steam in the pan, which prevents the skin from crisping and causes it to stick. If the fillets are thick, score the skin lightly with 2–3 shallow cuts to stop them curling in the pan.
12
Heat the Pan Until It Smokes (Don't Rush This)
Season the skin side of each fillet with fleur de sel and white pepper. Heat a non-stick or stainless steel pan over high heat until you see the first wisps of smoke rising from the surface — around 400°F / 200°C. The pan must be properly hot before the fish goes in. A cold or warm pan means the skin steams instead of sears.
13
Sear Skin-Side Down (Press for the First 10 Seconds)
Add the neutral oil to the hot pan, then lay the fillets skin-side down. Press each fillet gently with your fingertips or a spatula for the first 10 seconds to ensure full skin contact — this is what gives you an even, golden crust rather than a fillet that curls away from the pan. Reduce heat to medium-high and cook skin-side down for 3–4 minutes, watching the flesh turn opaque from the bottom upward. When the opacity reaches about 80% of the way up the fillet, the fish is nearly done.
14
Flip, Baste, and Rest (The Last 20% of Cooking)
Flip the fillets, add 30 g of butter to the pan, and tilt the pan to spoon the foaming butter over the flesh side for 30–45 seconds. Remove from the pan and rest for 1 minute on a warm plate — the residual heat will gently finish the last 20% of cooking without drying the fish out. The internal temperature should reach 140°F / 60°C for perfectly cooked, just-set white fish.
Assemble and Serve
15
Ladle the Broth and Add the Fresh Herbs
Ladle the hot broth and potatoes into four warm, wide bowls. Scatter the fresh chives, tarragon leaves, and basil generously over the broth. The heat of the soup will just wilt the edges of the herbs while keeping them vivid green and fragrant — do not stir them in or they will lose their color.
16
Add the Raw Garnish (At the Very Last Moment)
Divide the drained radish slices and julienned leek between the four bowls, placing them directly into the hot broth just before the fillet goes on. Added at the last second, they keep their crunch against the heat — adding a peppery, fresh bite that cuts through the richness of the broth and keeps every spoonful feeling alive.
17
Lay the Fillet on Top and Serve Immediately
Lay one seared fillet per bowl skin-side up across the top of the broth. Finish with a few drops of olive oil and a small pinch of fleur de sel on the skin. Serve immediately — the crisp skin stays intact for the first minute at the table, which is exactly when it matters. The fish appears twice in the bowl: invisible in the broth, then visibly beautiful on top.
Tips & Tricks
You're not sure how to ask the fishmonger for the bones: Say exactly: 'Can you fillet it for me and keep the bones and head please?' Most fishmongers do this without hesitation and it costs nothing extra. If they look uncertain, just tell them you're making stock — that always reassures them.
Your fumet is turning cloudy or bitter: The heat is too high. A rolling boil breaks proteins and fat into tiny particles that cloud the broth and pull out bitter compounds from the bones. You want a gentle trembling simmer — a bubble breaking the surface every second or two. If you see vigorous bubbling, reduce the heat immediately.
You forgot to watch the clock and the fumet has been going longer than 20 minutes: Strain it right away and taste. If it has gone bitter, there is no fix — fish bones release chalky, unpleasant compounds past the 20-minute mark. Set a timer the moment the simmer starts. This rule is non-negotiable.
The fish skin is sticking to the pan: Either the pan was not hot enough before the fish went in, or the skin was still damp. A properly hot pan with a thin film of oil will release the skin naturally after 2–3 minutes. If it is sticking, do not force it — wait another 30 seconds and try again. Forcing a stuck fillet tears the skin and ruins the presentation.
Your raw radish and leek garnish looks limp before it even hits the bowl: Soak the sliced radishes and julienned leek in ice-cold water for 10–15 minutes before serving. This firms them up and makes them curl slightly — a classic restaurant trick for keeping raw garnishes crisp and visually striking. Drain and pat dry just before they go into the bowl.
The broth tastes flat even after seasoning with salt: Fish broth almost always needs acid, not more salt. Add lemon juice in small increments — half a teaspoon at a time — tasting after each addition. You will notice the flavor lift and brighten immediately. A small splash of dry white wine or a few drops of fish sauce can also add depth.
Frequently Asked Questions
Is this a good summer meal soup to make for guests?
Yes — it is ideal for warm-weather entertaining. The broth and potatoes can be made up to 4 hours ahead and kept warm or gently reheated. When guests sit down, sear the fish fresh (about 5 minutes) and assemble the bowls at the table. The light, aromatic broth feels elegant without being heavy.
Which fish works best for this dish?
Sea bream and whiting are the top picks — both are affordable, widely available at fishmongers, and give a clean, sweet broth. Gurnard is an excellent underused option with exceptional bones. Avoid oily fish like salmon or mackerel, which will make the broth greasy and overpower the lemongrass aromatics.
Can I make the broth ahead of time?
Yes — the fumet and potato broth can be made up to 24 hours ahead and refrigerated. Reheat gently before serving. Do not add the fresh herbs, radishes, or julienned leek until the moment of serving, and always sear the fish fillets fresh. Pre-seared fish reheated in broth loses its texture entirely.
Can I use fish stock from a carton instead of making it from bones?
You can, but you will lose the central mechanic of the dish — the bones paying for the bowl. Carton stock also tends to be saltier and less clean-tasting. If you must substitute, use a good-quality fresh fish stock from a fishmonger, dilute it slightly, and still do the second lemongrass infusion with the potatoes.
Won't the raw radishes and leek go soggy in the hot broth?
Not if you add them at the very last moment — directly into the bowl just before the fillet goes on top. The brief contact with the hot broth softens their raw edge slightly, which is exactly what you want, but they keep their crunch for the first few minutes at the table. Slicing the radishes paper-thin and soaking both garnishes in ice water beforehand also helps them hold their texture longer.
Can I add other vegetables to the broth?
Keep it restrained — the lightness is the point. Thinly sliced fennel added with the potatoes works beautifully and echoes the anise note of the tarragon. Baby spinach wilted in at the end adds color. Avoid anything starchy or heavy like carrots or parsnips, which will muddy the clean broth.
What if I don't have a mandoline for the radishes?
Use the sharpest knife you own and slice as thinly as you can. The goal is paper-thin, uniform rounds — the thinner they are, the more delicate they feel in the bowl and the better they hold their texture against the hot broth. A sharp chef's knife works well with a little patience.
My broth tastes flat — how do I fix it?
Fish broth almost always needs acid, not salt. Add lemon juice in small increments, tasting after each addition — you will notice the flavor lift and brighten immediately. A small splash of dry white wine or a few drops of fish sauce can also add depth without making the broth taste one-dimensional.
