Heat the olive oil in a large skillet over medium heat. Add the chopped onion and diced bell pepper. Sauté for about 3-5 minutes, or until the vegetables are softened.
Add the ground beef to the skillet. Cook, breaking it apart with a wooden spoon, until it's browned and fully cooked, about 5-7 minutes. Drain any excess fat if necessary.
Stir in the pizza sauce, Italian seasoning, garlic powder, and sliced pepperoni. Mix well and let it simmer on low heat for about 5 minutes, allowing the flavors to meld.
Taste and season with salt and pepper as needed.
Spoon the sloppy joe mixture onto the hamburger buns. Top each with shredded mozzarella cheese.
If desired, place the assembled sandwiches back in the skillet, cover, and let the cheese melt for about 2-3 minutes.
Serve hot, and enjoy your Pepperoni Pizza Sloppy Joes!