Baked Fish Au Gratin recipe

1/4 cup butter
1 small onion, chopped
2 tablespoons all-purpose flour
1 teaspoon salt
1/2 teaspoon pepper
2 cups milk
2 pounds fish fillets (halibut, haddock or turbot), cut into serving-size pieces
1 cup shredded cheddar or Swiss cheese

Preheat oven to 350*. Grease a 13 x 9 x 2-inch baking dish; set aside.
Over medium heat melt butter in medium saucepan; add onion and saute until softened.
Stir in flour, salt and pepper and cook for 1 minute.
Gradually stir in milk.
Cook, stirring often, until thickened.
Remove from heat.
Pour a little sauce onto bottom of prepared baking dish.
Place fish in pan and pour remaining sauce over the fish.
Top with cheese and bake for 25 to 30 minutes or until fish flakes easily.
Makes 4 to 6 servings.