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 1/2-inch water.

Place peppers upside down in water, cover with foil, and bake for 20 minutes.

Heat olive oil in a large skillet over medium-high heat. Add onion and sauté for 3-4 minutes. Move onions to one side. Add ground beef and season with salt and pepper; sear for about 3 minutes.

Break up beef and toss with onions, cook for 2 minutes. Add garlic and cook for 1 minute until beef is cooked through. Drain excess fat.

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

Reduce oven temperature to 350°F. Turn peppers upright, sprinkle with salt inside, 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 until cheese melts and peppers reach desired tenderness, about 10-20 minutes. Garnish with parsley and serve warm.