Set aside 24 slices of pepperoni, and chop the remaining pepperoni into quarters.
Bring a large pot of lightly salted water to a boil. Add pasta and cook per package directions until al dente; drain.
In a medium skillet over medium-high heat, cook beef (or sausage) with onion until beef is brown. Drain any grease.
Stir together spaghetti sauce, ground beef mixture, 1 cup of mozzarella, chopped pepperoni, and cooked pasta in a 3 quart baking dish. Top with remaining mozzarella and the reserved pepperoni slices.
Bake for 25-30 minutes or until cheese is melted and the pepperonis are crisp.
Notes
Prepare pizza pasta bake up to a day ahead and pop it in the oven when ready!
Refrigerate leftovers in an airtight container for up to 3 days.
Reheat portions in the microwave.
Portions can be frozen for up to a month in containers or zippered freezer bags.