The fillets are poached in boiling salted water for a few minutes, till tender, well drained and arranged neatly on a dish. Cover with the following sauce: Put 1 oz. of chopped onion and the same of carrot in a small saucepan with 1 oz. of butter and simmer gently for 10 minutes. Remove from the saucepan and set aside. Put 2 ozs. of butter in the same saucepan, and when melted add 1½ of flour, and work to a smooth paste. Add gradually ½ pint of boiling milk, and keep stirring. Season with salt and pepper, add the onion and carrot, and simmer very gently for ¾ of an hour, without boiling. Meanwhile, shell the prawns, and pound the shells to a smooth paste in a mortar, with a little over half their weight in butter, and then rub through a sieve. Now strain the Béchamel sauce, put it in a saucepan and add the cream. Stir till it thickens. Remove from the fire and stir in the prawn butter, adding it gradually. When quite dissolved, add a dash of cayenne pepper, pour over the fish, and garnish with the cooked mussels, the prawns, and small fancy shapes of puff pastry.