Preheat oven to 425 degrees F. Make vol au vent by cutting a circle 8 inches in diameter in 1-inch thick pastry. Make another incision 1 1/2 inches from edge of pastry. Place round of pastry on a baking sheet brushed with cold water and bake 30 minutes. Remove lid and scoop out the inside to form a pastry case. Set aside. Place the mussels in a pan on a high heat with 1/4 cup of the dry white wine, cover, shake pan and allow to cook for 2 minutes. Allow to cool and then remove mussels from shells. Prepare fish stock by simmering snapper head with water, onion and Bouquet garni for 30 minutes and then strain. Place some clarified butter into a pan on the heat and add the shallot. Fry gently and then place the fish fillets on top and the scallops, and moisten with 1/2 cup of the fish stock and 1/4 cup of the dry white wine. Season with salt and pepper and poach for 5 minutes.
Sauce: Place the 1 tablespoon butter in a saucepan and when it has melted add the sifted flour. Stir gently to cook the roux but do not allow to color. Remove from the heat and add the warmed fish stock a little at a time, beating well after each addition. Place sauce back on heat and season with salt and pepper and allow to simmer 10 minutes, stirring from time to time. In a separate pan place a little clarified butter and when hot add the mushrooms. Season with salt, pepper and nutmeg and add a squeeze of lemon juice. Allow to fry gently for 1 minute. Remove the fish fillets to a dish and place the scallops, shrimp, mussels and mushrooms together in a bowl. Mix the yolk with the cream and then stir into the sauce - add 1 tablespoon butter and then fold in the seafood being very careful not to break the fish. Check the seasoning. Place the seafood mixture into the pastry case and heat in a 350 degree oven for 10 minutes.