Bring a large pot of water to a boil. Boil cabbage leaves 2 minutes; drain.
In large bowl, combine 1 cup cooked rice, egg, milk, onion, ground beef, salt, and pepper. Place about 1/4 cup of meat mixture in center of each cabbage leaf, and roll up, tucking in ends. Place rolls in slow cooker.
In a small bowl, mix together tomato sauce, diced tomatoes, brown sugar, lemon juice, and Worcestershire sauce. Pour over cabbage rolls.
Cover, and cook on Low 8 1/2 hours.
Preheat oven to 375 degrees, put crock in oven for 30 minutes, adding mozzarella cheese last 10 minutes, or till lightly browned.