1. Cook the pasta according to package instructions until al dente, then drain and set aside.
2. In a skillet over medium heat, cook the ground beef, Italian sausage, chopped onion, and minced garlic until the meat is browned, then drain excess grease.
3. Spray a 6-quart or larger slow cooker with nonstick cooking spray.
4. Spread a layer of pizza sauce on the bottom of the slow cooker.
5. Add a layer of cooked pasta, followed by a portion of the meat mixture.
6. Sprinkle mozzarella cheese on top, then add a layer of pepperoni and any optional toppings like bell peppers, mushrooms, or olives.
7. Repeat the layering process until all ingredients are used, finishing with a layer of cheese and additional pepperoni on top.
8. Cover with the lid and cook on low for 3-4 hours.
9. Once done, uncover and let the casserole sit for a few minutes before serving.
10. Serve hot with garlic bread and a side salad, if desired.
11. Store leftovers in an airtight container in the refrigerator or freeze for future meals.