Meanwhile, heat a deep 12-inch broiler-safe skillet over medium heat.
Cook sausage or ground beef, stirring to break up, until heated through but not browned. Drain fat and return to pan.
1 pound ground Italian sausage
Add onion, bell pepper, and garlic to skillet.
1 medium yellow or white onion, 2 green bell peppers, 4 cloves garlic
Season with salt and pepper.
Salt and pepper
Cover, and cook over medium heat until veggies are soft, about 4 minutes.
Stir in half pepperoni, crushed tomatoes, tomato sauce, tomato paste, and seasonings.
1 tablespoon Italian seasoning, 4 ounces sliced cooked pepperoni, 2-14.5 ounce cans crushed tomatoes, 8 ounce can tomato sauce, 6 ounce can tomato paste, 1 teaspoon dried sweet basil
Simmer until thickened, about 10 minutes.
Stir cooked pasta into sauce along with 1 cup mozzarella cheese; toss to combine.
2 cups shredded mozzarella cheese
Top with remaining half pepperoni and half mozzarella cheese.
Broil 1-2 minutes, or until cheese has melted.
Notes
Italian sausage - Hot or mild Italian sausage will work. You can also use ground beef if you prefer.
Onion - White or yellow onion works best.
Green bell pepper - Feel free to use another color of bell pepper.
Pepperoni - I recommend using regular pepperoni, not a reduced-fat pepperoni, like turkey pepperoni. The flavor of reduced-fat pepperoni tends to get lost in the casserole.
Tomato sauce - Tomato sauce is canned tomato puree.
Rotini pasta - Another type of pasta, like penne pasta or mafalda, can be used.