Separate the leaves from the cabbage being careful not to tear them. Drop cabbage leaves into boiling salted water; cover and cook for 3 minutes or until soft. Drain well.
Filling: combine beef, rice, onion, egg, salt and pepper. Mix well and spoon 1-2 Tbsp into each cooked cabbage leaf. Fold sides in, then roll up (secure with a toothpick if necessary). Place seam side down into a baking dish.
Sauce: combine tomato sauce, tomatoes, sugar, vinegar, and water and pour over cabbage rolls.
Bake covered at 350°F for 40 - 50 minutes.
Pour off tomato sauce into a sauce pan and bring to a boil. Stir in cornstarch and water mixture and cook until thickened. Pour sauce back over cabbage rolls and serve.