Heat a large skillet over medium heat and add a drizzle of olive oil. Once hot, add the diced onion and sauté for about 3-4 minutes until translucent. Add the minced garlic and cook for another 30 seconds, until fragrant.
Add the ground beef to the skillet. Cook the meat, breaking it up with a wooden spoon or spatula, until browned and cooked through. Drain any excess fat.
Stir in the pizza sauce, Italian seasoning, and red pepper flakes (if using) into the skillet with the beef. Let it simmer for about 5 minutes, allowing the flavors to meld.
Add the sliced pepperoni to the mixture, stirring until well combined. Season with salt and pepper to taste.
Toast the hamburger buns either in the skillet or toaster, if desired.
To assemble the Sloppy Joes, spoon the pepperoni pizza mixture generously onto the bottom half of each toasted bun. Sprinkle shredded mozzarella cheese on top and place the top half of the bun over the cheese.
Serve immediately while warm, and enjoy your Pepperoni Pizza Sloppy Joes!