Bring a large pot of lightly salted water to a boil. Add pasta and cook for 8 to 10 minutes or until al dente; drain.
In a medium skillet over medium-high heat, cook beef with onion until beef is brown. Drain. Combine beef mixture with spaghetti sauce, pepperoni and cooked pasta and pour into a 9x13 inch baking dish. Top with mozzarella.
Bake in preheated oven for 30 minutes, until cheese is melted and golden.