Fish Fillets in Wine

2 lb fish fillets
1 cup dry white wine
1 small onion, chopped
3/4 cup sliced mushrooms, if desired
Salt and freshly ground black pepper to taste
2 tbsp white sauce
1/4 cup cream
1 tbsp butter or margarine
1 tbsp chopped parsley

Preheat the oven to 350°F. Butter a baking dish. Arrange the onion and mushrooms on the bottom of the dish and sprinkle with salt and pepper.
Place the fish on top, add the wine and cover with buttered waxed paper or foil. Bake until the fish flakes easily when tested with a fork.
Drain the juices from the baking dish into a saucepan. Cook over a moderately high heat until reduced to about half.
Add the white sauce and cream and cook until sauce has thickened, rotating the pan all the time. Pour the sauce over the fillets and garnish with the chopped parsley.