Spray a 6-quart or larger slow cooker with cooking spray and set aside.

Cook the rigatoni according to package directions until al dente, drain, and set aside.

In a large skillet over medium-high heat, add the ground beef, ground Italian sausage, and diced onion. Cook until the meat is browned and the onions are softened.

Lower the heat to medium-low, add the pepperoni and minced garlic, and cook for an additional 5 minutes.

Drain excess grease, transfer the mixture to a bowl, and stir in the pizza sauce.

Add the cooked rigatoni to the mixture and stir until well combined.

Layer half of the pasta mixture in the slow cooker, then half of the shredded mozzarella. Repeat with the remaining pasta mixture and cheese.

Cover and cook on low for 3-4 hours until warmed through and the cheese is melted.

Serve topped with chopped parsley, if desired.