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Make the pie filling up to a day ahead (the sauce must be cold when you mix it with the other ingredients) but make the rosti and top the pie the day you want to bake it, otherwise the rosti will discolour.
Preheat the oven to 180°C, fan 160°C, gas 4. Put all the haddock in a roasting tin with the bay leaves, peppercorns and milk. Cover the tin with foil, bake for 20 minutes, then remove from the oven and leave to cool completely.
Meanwhile, to make the topping, parboil the potatoes (unpeeled) in boiling salted water for 10-12 minutes. You want them to be cooked but still firm enough to grate. Cool, then peel and coarsely grate the potatoes. Season and toss in a bowl with the melted butter and parsley.
Remove the fish from the milk and flake into big pieces into a large mixing bowl. Strain the cold milk through a sieve into a jug or bowl; discard the flavourings.
Put the milk, butter and flour in a pan and bring to simmering point, whisking constantly. Simmer until the sauce thickens, and season with salt, pepper and freshly grated nutmeg. Remove from the heat, stir in the cheese and, once melted, pour the sauce into the mixing bowl with the fish.
Stir in the prawns, parsley and peas, then tip into a large ovenproof baking dish. Top with the potato topping and bake for 50 minutes or until bubbling and golden on the top.
You can never have too many variations on fish pie, and this one combines two very British ingredients – smoked haddock and cheddar cheese