Melt margarine in a nonstick pan, cook mushrooms, peppers, onions, and garlic until softened (about 5 minutes).
Stir in the next 3 ingredients; simmer for 5 minutes.
Pour 1/2 of the tomato sauce mixture into a baking dish, add fish over sauce, and top with remaining sauce. Sprinkle evenly with mozzarella cheese. Bake until fish flakes easily when tested with a fork (about 12-15 minutes).