Cook and drain the noodles. Set aside.
Brown the ground beef and chopped onions together. Drain off the fat. Add the sauces to the meat and simmer for 5 to 10 minutes, stirring constantly until the ingredients are thoroughly mixed.
Layer the ingredients in a large crockpot in the following order: noodles, meat sauce, Cheddar cheese, mozzarella cheese, pepperoni. Repeat the layers a second time. Cover the pot with the lid and turn on LOW setting.
Serve when the cheese is melted. (If you are short on time, the HIGH setting can be used for approximately 30 minutes.)
Serves approximately 15.