Preheat oven to 350°F. Grease 9-inch glass baking dish. Melt butter in heavy large saucepan over medium-high heat. Add onion, bell pepper and basil and sauté until vegetables are tender, about 10 minutes. Stir in tomatoes. Season with salt and pepper and cook until saucelike, stirring constantly, about 5 minutes. Arrange fish in prepared dish. Pour sauce over. Sprinkle mozzarella. Bake until fish is cooked through and top is golden, about 25 minutes.
