In a medium saucepan scald the milk with 6 tablespoons of the butter, nutmeg, salt and pepper. Whisk in the flour, and continue whisking over a low heat until the dough detaches from the side of the saucepan. Remove the pan from the heat and beat in 1 of the egg yolks. Beat well. Add the remaining egg yolks, beating well between each addition. Set aside. Chop the mushrooms very fine and cook in 2 tablespoons of the butter. Set aside 1/2 pound of the John Dory fillets and cut them into matchstick-sized pieces. Beat the remaining fish into the flour mixture. Add salt and Cayenne. Mix well. Preheat the oven to 350F. Place the fish mixture in a large bowl. Set over a bowl of ice cubes. Beat the egg whites, one by one, into the mixture. Beat in the crÅme fraöche. Mix in the drained mushrooms, pistachios, and reserved fish. Butter a terrine dish. Pour the fish mixture into the terrine. Cover with a piece of buttered foil. Place the terrine in a roasting pan with 1 inch of boiling water. Bake for 1 1/2-2 hours. Serve the terrine hot with the melted butter and lemon sauce.
