Add the ground beef and onion to a skillet over medium high heat, and saute until the meat is browned and onions have softened.

Drain the excess fat.

Combine the pasta sauce, water, all but 16 of the pepperoni slices and 1 cup of the mozzarella cheese in the crockpot.

Mix in the pasta noodles, and stir to combine.

Add the garlic and salt and pepper to taste to the skillet with the meat and onions. Stir to combine, and saute another minute.

Pour the meat mixture into the crockpot, and stir to combine.

Top the mixture in the crockpot with the remaining mozzarella cheese and pepperoni slices.

Cook on low for 3 hours. Test the noodles at this point, and cook on low an additional hour if not tender.

Garnish with the parmesan cheese and serve.