Preheat oven to 400 degrees. Cook rice according to package instructions.

Trim about 1/4 inch from tops of bell peppers and remove stems, ribs, and seeds. Fill a baking dish with about 1/2 inch of water and place peppers upside down. Cover with foil and bake for 20 minutes.

Heat olive oil in a large skillet over medium-high heat. Add chopped onion and sauté for 3-4 minutes. Add ground beef, season with salt and pepper, and sear until browned, about 3 minutes.

Break up beef, add garlic, and cook until beef is cooked through, about 1 minute. Drain excess fat.

Stir in diced tomatoes, half the tomato sauce, cooked rice, parsley, and Italian seasoning. Season with salt and pepper to taste.

Reduce oven temperature to 350°F. Turn peppers upright, sprinkle inside lightly with salt, and fill with beef mixture. Pour remaining tomato sauce over peppers. Cover with foil and bake for 20 minutes.

Remove from oven, sprinkle with mozzarella cheese, and bake uncovered for an additional 10-20 minutes until tender. Garnish with parsley and serve warm.