Preheat your oven to 400°F (200°C). Place the halved bell peppers in a 9x13 inch baking dish. Drizzle with olive oil and season with salt and pepper. Bake for 15 minutes to soften.

While the peppers are baking, heat a large skillet over medium-high heat. Add the ground Italian sausage and cook until browned, breaking it up with a spoon. Add the diced onion and cook for 3-4 minutes until softened. Add the minced garlic and cook for another minute until fragrant. Drain off any excess grease.

Reduce the heat to medium. Stir the marinara sauce, 1 cup of the mozzarella cheese, half of the mini pepperonis, and the Italian seasoning into the skillet with the sausage mixture. Season with salt and pepper to taste. Let it simmer for 2-3 minutes.

Remove the peppers from the oven. Carefully spoon the meat mixture evenly into each pepper half.

Top the stuffed peppers with the remaining 1/2 cup of mozzarella cheese, the grated Parmesan cheese, and the rest of the mini pepperonis.

Return the baking dish to the oven and bake for 15-20 minutes, or until the filling is heated through and the cheese is melted, bubbly, and lightly golden. Let cool for a few minutes before serving.