Moisten fillets with a light sprinkle of water, then dip in flour. Set aside. Heat butter in frying pan, then add fillets and saute over medium for 10 minutes. Remove the fish and place in a casserole. In the same butter, saute onion until well browned. Add mushrooms, salt and pepper. Saute 5 minutes. Add grape juice and simmer for 5 minutes. Remove from heat and stir in the cream. Pour the sauce over the fish. Cover and bake at 350 degrees for 30 minutes.