1 onion, peeled, thickly sliced
750ml/1¼ pints whole milk
275g/9oz haddock fillet, pin bones removed, skin removed
300g/10oz smoked haddock fillet, pin bones removed, skin removed
300g/10oz salmon fillet, pin bones removed, skin removed
5 scallops, each cut in half horizontally to make two thinner discs
75ml/3fl oz dry white wine
3 tbsp chopped fresh chervil
40g/1½oz unsalted butter
40g/1½oz plain flour
salt and freshly ground black pepper
For the pie filling, preheat the oven to 200C/400F/Gas 6.
Bring the onion, cloves and milk to the boil in a large pan. Add the haddock, smoked haddock and salmon and reduce the heat until the mixture is simmering. Simmer for 4-5 minutes.
Add the scallops and cook for a further 1-2 minutes. Remove the pan from the heat and set aside to cool slightly.
Meanwhile, heat a large frying pan over a medium to high heat (you'll need a pan with a tight-fitting lid). When the pan is smoking, add the mussels and white wine and cover with the lid. Cook for 2-3 minutes, or until the mussels have opened. (Discard any mussels that have not opened completely.) Remove the pan from the heat.
Strain the cooked mussels through a sieve, reserving the cooking liquid in a bowl and discarding the shells.
Remove the mussel meat from the shells and place into an ovenproof dish. Flake the poached fish into the ovenproof dish. (Reserve the liquid the fish was poached in.) Sprinkle over the chervil, then stir carefully to combine.
Heat the butter in a separate pan over a medium heat. When the butter is foaming, add the flour and cook for 1-2 minutes, stirring continuously, until the mixture is thick, well combined and pale golden-brown. Remove the pan from the heat and whisk in the reserved liquids that the mussels and fish were cooked in, until the mixture is smooth and well combined.
Return the pan to the heat and cook, stirring continuously, for 1-2 minutes, or until the sauce has just thickened. Season, to taste, with salt and freshly ground black pepper.
Pour the sauce into the ovenproof dish to cover all of the fish. Carefully stir the ingredients in the dish until all of the fish is completely coated in the sauce.
Meanwhile, for the pie topping, boil the potato pieces in a pan of salted water for 12-18 minutes, or until tender. Drain well and return to the pan. Place the pan back on the heat and cook the potatoes for a few seconds so that any excess moisture evaporates.
Mash the cooked potatoes with a potato masher or ricer, then add 75g/3oz of the butter and all of the mustard and stir until smooth and well combined. Season, to taste, with salt and freshly ground black pepper.
Spoon the mashed potato in an even layer over the top of the pie filling to cover it completely. Place the dish onto a baking tray, then transfer to the oven and bake for 20-30 minutes, or until the potatoes are golden-brown and the filling is bubbling.
While the pie is cooking, bring a pan of salted water to the boil, add the peas and boil for 2-3 minutes, or until just tender. Drain well, then return the peas to the pan. Add the remaining 15g/½oz of butter and stir until all of the butter has melted. Season, to taste, with salt and freshly ground black pepper.
To serve, spoon the fish pie into the centre of six serving plates. Place a portion of the buttered peas alongside.
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