Wash cabbage and separate 12 large leaves. Drop into boiling salted water cover and cook 3 minutes. Drain and dry well. Combine beef, onions, oats, egg, salt, pepper, and milk, divide into 12 parts and place on center of cabbage leaves. Roll leaf around meat and fasten with toothpicks. Place in baking pan. Combine all ingredients for sauce; pour over rolls. Bake at 300 degrees for 1 1/2 hours, covered. Take out toothpicks, add 2 tablespoons cornstarch, 1/4 cup water and pour into sauce. Cook until thick.