Leek and Prawn Gratin

Preheat the oven to 200°C fan. Halve and slice the leeks, wash them in a large bowl of cold water, and drain well in a colander. Peel and devein the prawns, reserving the shells. Dice the shallot, mince the garlic, chop the tarragon and parsley, and grate the gruyère. Combine the breadcrumbs, 30g of the gruyère, the parsley, and olive oil in a small bowl; set aside.

Melt 20g of the butter in a small saucepan over medium heat. Add the reserved prawn shells and fry, stirring, for 3–4 minutes until deeply pink and fragrant.

Pour in the white wine and let it reduce by half, about 2 minutes, then add the fish stock and milk, bring just to a bare simmer, and steep off the heat for 10 minutes. Strain through a fine sieve, pressing the shells firmly to extract all liquid; discard the shells. You should have roughly 500ml of infused liquid.

Melt 30g of the butter in a wide, heavy-based sauté pan or braiser over low-medium heat. Add the shallot and a pinch of salt and sweat gently for 3 minutes until translucent. Add the garlic and cook for a further 1 minute until fragrant but not coloured.

Add the drained leeks to the pan with ½ teaspoon of the salt. Stir to coat in the butter, cover with a lid, and sweat over low heat for 20–25 minutes, stirring twice, until completely collapsed, jammy, and all their liquid has evaporated.

Push the leeks to the sides of the pan, melt the remaining 10g butter in the cleared centre, add the flour, and stir constantly for 90 seconds to cook out the raw flour taste.

Gradually ladle in the warm infused liquid, stirring continuously and incorporating the leeks as you go, until a smooth, thick sauce forms. Cook over medium heat, stirring, for 4–5 minutes until the sauce coats the back of a spoon.

Remove the pan from the heat. Stir in the Dijon mustard, tarragon, nutmeg, white pepper, and remaining salt. Fold in the raw prawns — the residual heat will just barely begin to turn them; they will finish cooking in the oven.

Transfer the mixture to a buttered 28cm gratin dish and spread level. Scatter the remaining 50g gruyère evenly over the surface, then top with the breadcrumb-parsley mixture.

Bake on the upper rack for 18–20 minutes until the topping is deep golden and the gratin is bubbling at the edges. Use an instant-read thermometer to confirm the prawns have reached an internal temperature of 63°C before serving.

Rest for 5 minutes before bringing to the table.