In a flameproof casserole or dutch oven, melt 3 tablespoons of the butter. Stir in the shallots, leek, and mushrooms and cook gently for 5 minutes, stirring occasionally. Sprinkle 3 tablespoons of the flour into the vegetables and stir to coat well. Cook for 2 minutes. Stir in the court bouillon and bouquet garni and cook for 10 minutes. In a small bowl whisk together the remaining butter and flour to form a paste. Drop the paste into the stock a little at a time and boil until the sauce thickens. Add the fish and mussels to the sauce and simmer for 10 minutes. In a small bowl whisk together the egg yolks, cream, and several drops of the lemon juice. Remove the pan from the heat and whisk in the cream mixture. Do not return pan to the heat. Serve immediately.
