Heat oven to 350 degrees. Brown meat and onion in a heavy skillet over low heat. Remove from heat. Mix in thoroughly celery, rice, horse-radish, mustard and egg. Cook cabbage leaves for 3 minutes in boiling salted water. Place meat mixture on cabbage leaves. Roll and fasten with toothpicks. Place close together in greased baking dish. Pour tomato puree and water over cabbage rolls. Cover and bake 30 minutes. Remove cabbage rolls. Pour sour cream into liquid remaining in baking dish. Serve over cabbage rolls.