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

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

Meanwhile, 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 the beef, toss with onions, cook 2 minutes. Add minced garlic and cook an additional minute. Remove from heat and drain excess fat.

Stir in diced tomatoes, half of the tomato sauce, cooked rice, parsley, Italian seasoning, salt, and pepper.

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

Remove from oven, sprinkle with mozzarella cheese, and bake uncovered until cheese melts and peppers are tender, about 10-20 minutes. Garnish with parsley and serve warm.