Special equipment: Oval gratin dish
Preheat oven to 375 degrees F.
Heat 2 tablespoons of oil in a large skillet over medium heat. Add the sausage and cook, stirring occasionally, until browned, about 6 minutes. (Don't fully cook the sausage, as it will be finished in the oven.) Drain the fat from the skillet.
Meanwhile, heat the remaining 2 tablespoons oil in a large skillet over medium-high heat. Add the onion and peppers and cook, stirring, until tender, about 2 minutes. Add in the remaining ingredients except for the mushrooms, cherry peppers, and Parmesan and cook, stirring occasionally, for another 3 minutes.
Combine the sausage, vegetable mixture, mushrooms, and cherry peppers in the gratin dish. Bake uncovered until well browned, about 40 minutes. Serve garnished with basil leaves and Parmesan.
Recipe courtesy George Stella