In a large skillet, cook the beef and onion over medium heat until meat is cooked. Add garlic. Stir in the tomato sauce, oregano, basil and salt. Add chopped pepperoni and bring to a boil. Reduce heat to low and simmer for 10 minutes. Remove from heat and set aside.
Preheat oven to 400 degrees, and line a baking sheet with parchment paper.
Spoon 2/3 cup of sloppy Joe filling on the bottom half of each bun. Top with shredded mozzarella and 3 pieces of pepperoni. Bake for 3 to 5 minutes, or until cheese melts. Top with bun and serve!