Cut tops off peppers and remove seeds; set aside. In a large bowl, combine the beef, mozzarella cheese, rice, onion, garlic, parsley, salt and pepper; spoon into peppers. Transfer to an oval 5- or 6-qt. slow cooker. Pour tomatoes over peppers; pour broth around the sides.
Cover and cook on low for 4-5 hours or until a thermometer reaches 160° and peppers are tender. Sprinkle with Parmesan cheese and additional parsley.