Preheat oven to 425°. In a saucepan, cook sausage, onion and green pepper over medium heat until meat is no longer pink; drain. Stir in 2 tablespoons flour, basil and fennel. Add tomato sauce. Bring to a boil; cook and stir for 2 minutes.
Transfer to an ungreased 13x9-in. baking dish. Sprinkle with mozzarella cheese. Place remaining 1 cup flour in a large bowl. In another bowl, whisk eggs, milk, oil, salt and, if desired, Parmesan cheese. Stir into flour until blended; pour over casserole.