
Preheat the oven to 375°F. Lightly grease a 9×13 baking dish.
Slice the tops off the bell peppers and remove seeds and membranes. Arrange peppers upright in the baking dish. Remove the stems from the tops of the peppers and dice. Set aside.
Heat olive oil in a large skillet over medium heat. Add onion, reserved chopped peppers and garlic, cooking until softened, about 3 minutes.
Add ground beef to the skillet. Cook until browned, breaking it up with a spoon, about 6–8 minutes.
Stir in cooked rice, tomato sauce, Italian seasoning, salt, and black pepper. Cook for 2–3 minutes until well combined. Remove from heat and stir in ½ cup mozzarella and all the Parmesan.
Spoon the beef and rice mixture evenly into the prepared peppers.
Cover the dish with foil and bake for 30 minutes. Remove foil, sprinkle the tops with the remaining ½ cup mozzarella, and bake uncovered for 10–15 minutes, until the cheese is melted and bubbly.
Garnish with fresh parsley and serve warm.