Fish pie

While the potatoes cook, pour the milk into a large sauté pan. Add the white fish fillets and bring it to a boil. Immediately take the pan off the heat, cover it, and let the fish stand in the hot milk for 3 minutes. Carefully lift the fish out, reserving the milk for the sauce. Pat the fish dry with kitchen paper, then gently flake it into an ovenproof dish, making sure to remove any skin or bones.

In a clean pan, warm the remaining olive oil. Stir in the plain flour and cook for about 30 seconds to make a roux. Gradually whisk in 200–250ml of the reserved poaching milk (you can discard any leftover milk). Grate in some fresh nutmeg, season with salt and pepper, and let it bubble gently until the sauce is visibly thick. Stir in the 3 tablespoons of double cream until smooth.

Preheat your oven to 190°C (fan 170°C / gas 5). Grate the Jerusalem artichokes and add them to the dish with the flaked fish, along with the finely sliced leek and raw prawns. Stir the lemon zest and juice into the prepared sauce, then pour the sauce over the fish mixture. Gently combine everything with a wooden spoon.

Spoon the mashed potatoes over the fish filling, then use a fork to create little peaks and swirls — these will crisp and brown beautifully in the oven. Sprinkle the grated Gruyère cheese evenly over the top. Bake for 35–40 minutes, until the topping is golden and the filling is bubbling around the edges. Serve hot, perhaps with some simply wilted greens.