Butter an oven dish. Season the fish with salt and pepper and place it in the buttered dish. Spread with the shallots, onions, parsley, garlic and mushrooms. Dot the fish well with 3 tablespoons of the butter and pour the wine over it.
Bake for 20 to 30 minutes, basting frequently. Remove the fish to a heated platter.
Add the remaining 1 tbsp of cold butter to the sauce in the baking dish to bind it. Do not heat.
Pour the sauce over the fish without straining. Decorate, if desired, with lemon slices and additional chopped parsley.