Use a 4-quart slow cooker. Place the diced onion, sausage, and garlic into the bottom of the slow cooker. Add in the entire can of diced tomatoes, and the Italian seasoning. Pour in the noodles (rinse them in a colander before adding to the pot). Stir in water. Top with shredded mozzarella cheese and pepperoni.
Cover and cook on low for 4-6 hours, or until the edges have browned and pulled away from the edge of the pot and the pasta is bite tender.