Preheat oven to 350º Fahrenheit.
Brown ground beef and onions over medium heat until meat is cooked thoroughly.
Cook rotini noodles according to package directions.
In a large bowl, mix together spaghetti sauce and cooked ground beef.
Add noodles and stir again.
In a greased 9×13-inch baking dish, layer half the noodles, half the pepperoni and half the cheese. Repeat.