Cardoon and Prawn Gratin

Remove the tough outer strings from the cardoon stalks by peeling with a vegetable peeler or sharp knife. Cut into 6cm batons and immediately rub the cut edges with the lemon halves to prevent oxidation. Fill a large pot with 3 litres of water, squeeze in the remaining lemon juice, add the flour-water blanc mixture and 1 tsp salt, and whisk to combine. Add the cardoon pieces to the blanc.

Bring the cardoon pot to a boil over high heat, then reduce to a steady simmer and cook for 35-40 minutes until the cardoons are completely tender when pierced with a skewer — they should have no resistance. Drain and set aside on a clean towel.

While the cardoons cook, simmer the reserved prawn shells with the white wine and 100ml water in a small saucepan over medium heat for 15 minutes, pressing down on the shells. Strain the prawn shell stock through a fine sieve, pressing firmly, and discard the shells. You need approximately 120ml of stock.

In a medium saucepan over medium heat, melt 2 tbsp of the butter until foaming. Add the shallot and sweat for 3-4 minutes until completely soft and translucent, with no colour.

Add the 2 tbsp plain flour to the shallot pan and stir continuously with a wooden spoon for 2 minutes to cook out the raw flour — the roux should smell lightly nutty.

Remove the roux from the heat and gradually whisk in the warm prawn stock, followed by the milk, adding each liquid in a thin stream and whisking vigorously to prevent lumps. Return to medium heat.

Whisk the sauce continuously for 6-8 minutes until it thickens enough to coat the back of a spoon. Stir in the double cream, Dijon mustard, nutmeg, white pepper, and half the parsley. Taste and adjust seasoning.

In a separate frying pan, heat the olive oil and remaining 1 tbsp butter over high heat until the foam subsides. Add the prawns in a single layer and cook for 90 seconds per side until pink and just opaque — they will finish in the oven. Remove immediately and use a meat thermometer to confirm an internal temperature of 63°C.

Heat the oven to 200°C fan. Butter a large gratin dish and arrange the drained cardoon pieces in an even layer. Scatter the par-cooked prawns over the cardoons. Pour the sauce evenly over the top, ensuring full coverage.

Combine the Gruyère and Parmesan and scatter evenly over the surface. Bake for 18-20 minutes until the gratin is bubbling at the edges and the cheese crust is deep golden and blistered in places.

Rest the gratin for 5 minutes before serving, then scatter the remaining chopped parsley over the top.