First, prepare the sauce for the filling. Heat the butter in a pan and sweat the shallots until soft. Add the wine and Noilly Prat vermouth and reduce by half. Add the stock and bubble away again until reduced by half. Pour in the cream and boil until reduced to a thick sauce consistency. Strain through a sieve and discard the shallot. Stir in the mustard, if using, and check for seasoning, adding a little lemon juice to taste. Leave to cool completely.
Next, make the mash topping. Put the potatoes into a pan of salted water and bring to the boil. Cook for 15–20 minutes until tender when pierced with a knife. Drain well and push them through a potato ricer. Mix in the butter and hot milk until melted and well incorporated. Season to taste and leave to cool slightly. Beat the egg yolks lightly, then mix into the cooled mash. Set aside while you prepare the fish.
Heat the oven to 180°C/Gas 4. Check the fish fillets for any small bones, removing any with tweezers, then cut into 3cm chunks. Season lightly, then gently fold the fish, oysters and scallops into the sauce with the basil leaves. Transfer to a 1.75–2-litre ovenproof baking dish. Spread the mash over the filling and run a fork over it for a rustic finish. Grate over a layer of parmesan. Bake for about 25–35 minutes until the pie is bubbling and golden brown on top.
From Cooking for Friends, by Gordon Ramsay, (HarperCollinsPublishers)
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