Pizza Casserole

Preheat oven to 350°. Grease a 13"-by-9" glass baking dish with oil.

Bring a large pot of water to a boil over high heat. Add pasta and 2 tablespoons salt and cook, stirring occasionally, until al dente, 7 to 9 minutes (pasta should still be slightly undercooked in the center). Drain pasta; set aside.

Meanwhile, in a large skillet over medium heat, heat 1 tablespoon oil. Add sausage and cook, breaking into bite-size pieces, until browned and cooked through, 4 to 5 minutes. Transfer sausage to a medium bowl, leaving as much fat in skillet as possible.

In same skillet over medium heat, combine mushrooms and 1 tablespoon oil. Cook, tossing occasionally, until browned, 5 to 6 minutes. Season with 1/2 teaspoon salt, then transfer mushrooms to bowl with sausage.

In same skillet over medium heat, cook onion, garlic, 1/2 teaspoon salt, and remaining 2 tablespoons oil, stirring occasionally, until onion is softened, 5 to 7 minutes. Stir in tomato sauce, diced tomatoes, oregano, basil, red pepper flakes, and remaining 1 teaspoon salt. Remove 2 cups sauce and reserve. Fold bell peppers and cooked sausage, mushrooms, and reserved pasta into remaining sauce. Cook, stirring, until heated through, 1 to 2 minutes more.

Pour half of pasta mixture into prepared casserole dish. Sprinkle with 1 cup mozzarella. Top with remaining pasta mixture. Pour reserved 2 cups sauce over. 

Cover pan with foil and bake until hot and bubbling around the edges, about 20 minutes. Remove foil, top with remaining 2 cups mozzarella, then pepperoni, and continue to bake until cheese is melted and pepperoni is browned, 15 to 20 minutes more.