Preheat oven to 350°F.
Generously butter au gratin or similar shallow dish.
Arrange fish fillets in dish.
Melt butter in small saucepan over low heat.
Add mushrooms and saute until softened.
Add grapes, stirring gently .
Add remaining ingredients and blend well.
Spoon over fish.
Tent with foil and bake until fish is flaky and sauce is heated through, about 20 minutes.