In a large skillet over medium heat, cook the ground beef and diced onion together until the beef is fully browned and the onion becomes soft, approximately 5–6 minutes. Drain excess fat if necessary.

Stir in the minced garlic and cook for 1 minute to release its aroma and flavor.

Add the pizza sauce, chopped pepperoni, Italian seasoning, and optional red pepper flakes to the skillet. Stir well and let the mixture simmer for 5–7 minutes until it thickens slightly.

Stir in the shredded mozzarella cheese until it melts and is fully incorporated into the meat mixture.

While the filling simmers, heat another skillet over medium heat. Spread olive oil or butter on the hamburger buns and toast them until golden brown.

Spoon the pepperoni pizza sloppy joe mixture generously onto the toasted buns. Garnish with grated Parmesan and chopped parsley if desired. Serve immediately while hot!