Pre-heat the oven to gas mark 6, 400°F (200°C).
First of all arrange the haddock in a baking tin, pour over the milk and add the bay leaf, peppercorns and parsley stalks, then bake, uncovered, on a high shelf of the oven for 10 minutes. Meanwhile, remove the skin from the kipper fillets and skin and bone the Arbroath smokies – the flesh will come off very easily. Then chop them into 2 inch (5 cm) pieces, along with the smoked salmon, if the slices are whole, then place all the prepared fish in a mixing bowl. Next, when the haddock is cooked, strain off the liquid and reserve it, discarding the bay leaf, parsley stalks and peppercorns. Then, when the haddock is cool enough to handle, remove the skin and flake the flesh into largish pieces, adding it to the bowl to join the rest of the fish.
Next make the sauce. Do this by melting the butter in the saucepan, stir in the flour and gradually add the fish liquid bit by bit, stirring continuously. When all the liquid is in, finish the sauce by gradually adding the single cream, then some seasoning, and simmer for 3-4 minutes, then stir in the chopped parsley. Now add the hard-boiled eggs, capers and cornichons to the fish, followed by the lemon juice and, finally, the sauce. Mix it all together gently and carefully so as not to break up the fish too much, then taste and check the seasoning and pour the mixture into the baking dish.
Now, to make the topping, peel and quarter the potatoes, put in a steamer fitted over a large saucepan of boiling water, sprinkle with a dessertspoon of salt, put a lid on and steam until they are absolutely tender – about 25 minutes. Then remove the potatoes from the steamer, drain off the water, return them to the saucepan and cover with a clean tea cloth to absorb some of the steam for about 5 minutes. Now add the butter and crème fraîche and, on the lowest speed, use an electric hand whisk to break the potatoes up, then increase the speed to high and whip them up to a smooth, creamy, fluffy mass. Taste, season well, then spread the potatoes all over the fish, making a ridged pattern with a palette knife. Now finally sprinkle over the grated cheeses and bake on a high shelf in the oven for 30-40 minutes, or until the top is nicely tinged brown. Serve each portion garnished with the watercress.
