In a large skillet over medium-high heat, add olive oil, ground beef, and 1/2 cup of the mini pepperoni. Cook, breaking up the beef, until browned and pepperoni is slightly crisp, about 5-7 minutes.

Add the diced onion and bell pepper to the skillet. Cook for 5-6 minutes until softened. Add the minced garlic and cook for 1 more minute until fragrant.

Reduce heat to medium. Stir in the tomato sauce, tomato paste, water, brown sugar, Worcestershire sauce, oregano, basil, fennel seeds, and red pepper flakes (if using). Mix until fully combined.

Bring the mixture to a simmer and cook for 10-15 minutes, stirring occasionally, until the sauce has thickened to a Sloppy Joe consistency. Season with salt and black pepper to taste.

While the filling simmers, lightly toast your buns either in a toaster or cut-side down in a dry skillet.

Preheat your oven's broiler on high. Place the toasted buns on a baking sheet. Spoon the filling onto the bottom buns, then top with shredded mozzarella and the remaining mini pepperoni.

Place the baking sheet under the broiler for 1-3 minutes, watching closely, until the cheese is melted and bubbly.

Remove from oven, sprinkle with Parmesan and fresh herbs. Top with the other bun half and serve immediately.