1. Preheat oven to moderate (350° F.).
2. Wipe the fish with a damp cloth and dredge with flour seasoned with salt and pepper. Place the fish in a buttered baking dish and dot with butter. Bake twenty minutes, basting often.
3. Sprinkle the fish with the mushrooms and onion and add the bay leaf, garlic and white wine. Cover with buttered waxed paper or aluminum foil (butter side down) and bake twenty minutes longer, basting occasionally.
4. Transfer the fish, mushrooms and onion to a heated platter and discard the bay leaf and garlic. To the liquid remaining in the pan add the heavy cream. Heat the sauce to the boiling point but do not let it boil. Correct the seasonings and pour over the fish. Garnish with chopped parsley.