Drop cabbage leaves into boiling salted water; cover and cook for 3 minutes. Drain well. For filling, combine ground beef, rice, onion, egg, and salt, pepper, and milk. Mix well and divide into 12 portions. Place a portion into the center of each cabbage leaf. Roll leaf around filling; fasten with toothpick. Place in a baking dish.
For sauce, combine tomato sauce, tomatoes, sugar, vinegar, and 1/2 cup of water and pour over cabbage rolls. Bake covered in a preheated 350° oven 40 to 45 minutes. Remove rolls and discard toothpicks. Place pan with juices over medium heat or transfer the juices to a saucepan and place over heat, bring to a boil and cook until thickened.