1. Preheat the oven to 375°F and lightly grease a baking dish. Cut the tops off the bell peppers, remove the seeds and membranes, and set the peppers upright in the dish.

2. Heat the olive oil in a large skillet over medium heat. Add the diced onion and cook until softened, then stir in the garlic and cook for about 30 seconds.

3. Add the ground beef and cook until browned, breaking it apart as it cooks. Drain any excess grease.

4. Stir in the cooked rice, half of the tomato sauce, Italian seasoning, salt, and black pepper. Mix until everything is evenly combined and heated through.

5. Spoon the filling into each bell pepper, packing it gently. Pour the remaining tomato sauce around and over the peppers.

6. Cover the dish with foil and bake for 30 minutes. Remove the foil and bake for another 10 to 15 minutes, until the peppers are tender.

7. Sprinkle the mozzarella cheese over the tops and return to the oven for 5 minutes, until melted and bubbly.

8. Garnish with chopped parsley and serve warm.