Put the cod into a large saucepan, dust with court-bouillon, moisten with cold water. Salt lightly. Bring to the boil then turn down the heat immediately to cook over a very low heat for 10 minutes. Turn off the heat. Allow to poach for 5 minutes. Drain. Discard all bones and skin. Divide into egg-sized pieces. Make a bÄchamel with 1/3 milk, 1/3 cream and 1/3 court-bouillon, add 2 tablespoons of grated GruyÅre. Put the fish into an ovenproof dish, coat with the sauce, sprinkle with grated cheese, add some small pieces of butter. Add the raw mussels. Bake in a hot oven (400F) to gratin slowly.
