Seafood Gratin

Dice and slice all the vegetables as directed. Combine the breadcrumbs with the olive oil and half the Gruyère in a small bowl and set aside. Bloom the saffron in 2 tablespoons of warm water for 10 minutes.

Combine the white wine, fish stock, reserved prawn shells, bay leaf, and thyme in a wide saucepan over medium-high heat and bring to a simmer. Cook for 10 minutes until the liquid is fragrant and slightly reduced.

Strain the poaching liquor through a fine sieve, discard the solids, and return the liquid to the pan over medium heat. Add the mussels, cover, and cook for 3–4 minutes until all shells have opened — discard any that remain closed. Remove the mussels with a slotted spoon, pick the meat from the shells, and set aside, keeping the poaching liquor.

Reduce the heat to a gentle simmer and poach the fish chunks in the liquor for 3 minutes until just opaque and reaching an internal temperature of at least 63°C, checked with a meat thermometer — they should flake when pressed lightly. Remove with a slotted spoon and set aside.

Add the prawns to the same liquor and poach for 2 minutes until pink and just cooked through, reaching an internal temperature of 63°C when checked with a meat thermometer. Remove and set aside.

Add the scallops to the liquor and poach for 90 seconds per side until just firm and opaque at the edges, reaching 63°C internally, checked with a meat thermometer. Remove and set aside. Reserve all the poaching liquor — you need approximately 400ml; reduce or top up with extra fish stock as needed.

Melt the butter in a heavy-based saucepan over medium heat. Add the onion, shallots, and leek and cook, stirring, for 6–7 minutes until completely softened and translucent but not coloured, then add the garlic and cook for 1 minute more until fragrant.

Add the flour and stir constantly for 2 minutes to cook out the raw flour taste — the roux should smell lightly nutty.

Add the reserved poaching liquor in a slow, steady stream, whisking constantly to prevent lumps, then whisk in the double cream and bloomed saffron with its soaking water.

Simmer the sauce over medium-low heat, stirring frequently, for 8–10 minutes until thick enough to coat the back of a spoon heavily. Season with salt, white pepper, and lemon zest, then fold in the remaining Gruyère, parsley, and chives.

Preheat your grill to its highest setting. Gently fold all the seafood into the sauce, taking care not to break up the fish, then divide the mixture evenly among four individual gratin dishes or one large baking dish.

Scatter the breadcrumb and cheese mixture evenly over the top of each dish, then grate over the Parmesan.

Place under the grill for 4–5 minutes until the topping is deep golden brown and bubbling fiercely at the edges. Serve immediately with lemon wedges.