1. Preheat oven to 350°.
2. Grease 3 quart glass baking dish.
3. Saute onion and pepper in oil over medium heat.
4. Add basil and cook until tender, about 10 minutes.
5. Stir in tomatoes, salt and pepper to taste, and heat through.
6. Arrange fish in baking dish.
7. Pour sauce over fish and sprinkle with mozzarella.
8. Bake 25 minutes or until fish is white and flaky.