Baked Flounder Supreme

1 can (4 oz.) sliced mushroom
6 large flounder fillets, skinned
to taste: seasoned salt (see below)
6 slices Swiss cheese, folded in half
1 large tomato, peeled, cut into 6 slices
2 small onions, sliced
2 tbsp. melted butter or margarine
1-1/2 tbsp. flour
1 cup half-and-half
1/4 cup + 2 tbsp. dry sherry
enough: hot cooked rice (for 6)

    Heat oven to 400ºF (200ºC).  Drain mushrooms, reserving liquid.  Add enough water to liquid to make 1/2 cup; set aside.  Sprinkle fillets on all sides with seasoned salt; starting at wide end, roll up each fillet.  Place, seam side down, in a greased 13x9" baking dish.  Place a cheese slice, topped with tomato slice between each roll; set aside.  Sautè onion and mushrooms in butter until golden.  Stir in flour, 1-1/2 tsp. seasoned salt, half-and-half, mushroom liquid, and sherry.  Bring to a boil, pour over fish.  Bake 20 minutes or until fish flakes easily when tested with a fork.  Serve with hot cooked rice.  Makes 6 servings.