Boil water in a large pot and add the short pasta. Cook until just al dente, around 6-7 minutes. Drain the pasta and set aside to cool a bit.

Brown the ground Italian sausage and ground beef in a skillet over medium heat for about 8 minutes, stirring occasionally. Once fully cooked, drain excess fat and set the meat aside.

Microwave the sliced pepperoni on a plate lined with paper towels for 15 seconds to reduce greasiness.

Mix the cooked pasta, browned meats, pepperoni, chopped onion, pizza sauce, Italian seasoning, and chopped garlic in a large mixing bowl. Stir until everything is well combined and coated with sauce.

Layer the mixture in a greased 5-quart slow cooker, starting with half of the pasta mixture, followed by a layer of shredded mozzarella cheese. Repeat the layers, finishing with mozzarella on top.

Cover the slow cooker and cook on LOW for 3.5 to 4 hours, checking occasionally.