Preheat oven to 375°F and lightly grease a 9x13-inch baking dish.
Heat olive oil in a large skillet over medium heat and sauté onion until softened, about 5 minutes, then add garlic and cook 1 minute more.
Add ground meat to the skillet and cook until browned, about 8 minutes, breaking it up as it cooks.
Stir in cooked rice, diced tomatoes, Italian seasoning, salt, pepper, and 1 cup of mozzarella cheese until well combined.
Arrange the hollowed bell peppers upright in the prepared baking dish and fill each with the meat and rice mixture.
Cover the dish with foil and bake for 35 minutes, then remove foil, top peppers with remaining cheese, and bake uncovered for 10 more minutes until cheese is melted and bubbly.
Garnish with fresh parsley before serving.