Prepare ahead

Step 1 can be completed up to a day in advance. The fish can also be poached 1 day in advance; cover and chill the fish and reserve the milk until needed. The prawns can be shelled and deveined several hours in advance, and chilled until needed.

1. Boil the potatoes in a large saucepan of lightly salted water about 15 minutes, or until tender. Drain well. Mash until smooth, then beat in 1/3 cup milk and 3 tbsp butter. Season with salt and pepper.

2. Meanwhile, combine the haddock and remaining milk in a large frying pan. Simmer about 10 minutes until the fish flakes easily. Use a slotted spoon to transfer the fish to a plate. Add the shrimp shells (reserve the shrimp), peppercorns, and bay leaf to the milk and simmer over very low heat for 10 minutes.

3. Preheat the oven to 425°F (220°C). Melt 4 tbsp butter in a saucepan over medium heat. Whisk in the flour and cook for 1 minute. Strain the milk mixture into the saucepan and whisk well. Cook, whisking often, until the sauce comes to a boil and thickens.

4. Stir in the cream, parsley, and lemon juice, and season with the cayenne, salt, and pepper. Flake in the fish and add the reserved shrimp.

5. Spoon the fish mixture into a baking dish, top with the mashed potatoes, and dot with the final 1 tbsp butter. Place on a baking sheet. Bake for 20–25 minutes, or until the topping is golden and the filling is hot when you stick a knife into it.