Preheat the oven to 375°F (190°C). Lightly grease a 9×13-inch baking dish.
Slice the tops off the peppers, remove the seeds and membranes, and trim the bottoms slightly if needed so the peppers stand upright.
Heat the olive oil in a large skillet over medium heat. Cook the onion for about 4 minutes, until softened.
Add the garlic and cook for 30 seconds, until fragrant.
Add the ground beef and cook until browned, breaking it apart as it cooks. Drain the excess grease.
Stir in the tomato paste, 1½ cups of the tomato sauce, Italian seasoning, paprika, oregano, salt and black pepper. Simmer for about 5 minutes.
Stir in the cooked rice until evenly combined.
Spoon a little of the remaining tomato sauce into the bottom of the prepared baking dish.
Fill each pepper generously with the beef and rice mixture and stand the peppers upright in the dish.
Spoon the remaining tomato sauce over the tops of the peppers.
Cover the dish with foil and bake for 35 minutes.
Remove the foil, sprinkle the mozzarella over each pepper, and bake uncovered for 10–15 minutes, until the cheese is melted and lightly golden.
Garnish with fresh parsley and serve hot.