Preheat the oven to 375°F and lightly grease a baking dish.
Heat olive oil in a large skillet over medium heat. Add onion and garlic, cooking until fragrant.
Stir in the ground beef or turkey, breaking it apart as it browns. Cook until no longer pink.
Mix in the diced tomatoes, cooked rice, Italian seasoning, salt, and pepper. Simmer for about 5 minutes.
Spoon the mixture into the prepared bell peppers and place them upright in the baking dish.
Top each pepper generously with mozzarella cheese.
Cover the dish with foil and bake for 30 minutes. Remove foil and bake another 10 minutes until cheese is melted and bubbly.
Garnish with fresh parsley and serve warm.