Brown ground beef and onion in a large, oven proof skillet and drain excess fat.
Add 25 slices of pepperoni and spaghetti sauce.
Add pasta and water, stir and bring to a boil.
Reduce heat, cover and simmer for 15 minutes until the most of the liquid is absorbed and pasta is tender.
Remove lid and sprinkle top with cheese and remaining pepperoni.
Broil until the cheese is lightly browned.