Bake fish fillets according to package directions.
Meanwhile, in a medium saucepan cook mushrooms in hot oil until tender. Stir in cornstarch; add undrained tomatoes and pepper. Cook and stir until mixture is thickened and bubbly. Cook and stir for 2 minutes more. Remove from heat.
To serve, spoon tomato mixture over fish. Sprinkle with Parmesan cheese. Serve with pasta or rice. Makes 6 servings.