Pizza Casserole

1 1/2 pounds ground beef
1 large onion, chopped
2/3 pound rigatoni noodles, slightly undercooked
28 ounces pizza sauce
2 pounds shredded mozzarella cheese
20 pepperoni slices, cut in half

Brown meat and onions in skillet; drain. Layer ingredients in crock pot in the order given. Cook on LOW for 4 to 6 hours.