Put the coley and haddock into a large pan and pour boiling water over just to cover. Simmer very gently for 10 minutes. Turn it all out into a clean sink and leave until the fish is cool enough to handle. Then remove skin and any bones. Flake the fish onto a plate and reserve.
Wash the spinach thoroughly and put in a saucepan--do not add any water. Cook gently until it collapses, then raise the heat and cook until done, about 3 minutes more. Drain well, pressing the liquid out. Return the spinach to the saucepan with 2 ounces butter and season to taste with salt, pepper and nutmeg. Spread the spinach evenly on the bottom of a well-buttered oven dish large enough to take the rest of the ingredients.
Melt the remaining 2 ounces of butter in a saucepan and cook the onion gently until translucent. Add the flour and stir around to make a roux. Have the milk heated to a simmer and add it little by little until you have a smooth bechamel sauce. Pop in the bay leaf and let it simmer for 1/2 hour, stirring occasionally.
Stir the anchovy essence and parsley into the sauce and season with salt and pepper. Mix in the flaked fish and pour onto the spinach. Slice the whites of the scallops in half horizontally and distribute over the fish, interspersed with their corals. Scatter the prawns over the top, sprinkle with the Parmesan and dot with a little extra butter. Bake in a preheated oven at 400 degrees for 20 to 30 minutes, until heated through and browned on the top. Serve with a puree of potato and a tomato salad.