Preheat oven to 350 degrees F (180 degrees C).. Coat a 9x13 baking dish with cooking spray and set aside. Boil a pot of salted water and cook the Pasta (1 package) to al dente.
Shred the Mozzarella Cheese (2 cup). Chop the Yellow Onion (1).
Cook Ground Beef (1 pound) in a large skillet over medium-high heat until meat is no longer pink. Drain excess grease and add the chopped onion to the skillet.
Cook over medium heat for an additional 3 to 5 minutes, or until onion is soft and fragrant.
Add Salt (to taste) and Ground Black Pepper (to taste), along with the Pasta Sauce (1 jar) and cooked pasta. Cook until sauce is warm, about 5 additional minutes.
Add skillet contents to the prepared baking dish.
Top with mozzarella cheese and Pepperoni Slice (20 slice).
Bake in the preheated oven for 30 minutes.