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Preheat the oven to 200°C/Gas 6. Grease a shallow (about 2 litre capacity) pie dish.
Start by making the mashed potato for the topping. Chop the potatoes into chunks and cook in boiling salted water until tender. Drain well and push through a potato ricer, or mash until smooth. Add the butter and hot milk and mix until well incorporated. Allow to cool slightly, then stir in the egg yolks. Season well and put to one side.
Sauté the shallots or onion in the oil and butter with the thyme leaves for about 5 minutes until softened. Add the Noilly Prat and Pernod (if using), then cook for 4–5 minutes until reduced right down.
Stir in the flour and cook for a minute or so. Heat the stock in a small pan or a jug in the microwave. Gradually stir it into the shallot mixture with a wooden spoon until smooth, and boil for about 5 minutes until reduced by a third. Mix in the milk, lower the heat and simmer for a few minutes. Season well, then add the cream and parsley.
Meanwhile, cut the salmon and cod into bite-sized chunks and scatter in the pie dish with the scallops and prawns. Sprinkle with the lemon juice and seasoning. Put the dish on a baking sheet.
Pour over the sauce and mix well with a fork. Spread the mashed potato on top and fluff up with a fork. Scatter with the grated cheese and put the pie immediately in the oven. Bake for 10 minutes, then turn the oven down to 180°C/Gas 4, and bake for another 20 minutes, turning if it browns unevenly. Allow to stand for 10 minutes before serving.