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.
Add onion, bell pepper, and garlic to skillet.
Season with 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.
Simmer until thickened, about 10 minutes.
Stir cooked pasta into sauce along with 1 cup mozzarella cheese; toss to combine.
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.