Mix together, lightly and thoroughly, the ground beef, salt, pepper and egg. Mix in the cooked rice and chopped onion. Steam the cabbage or place in boiling salted water, then drop in cold water. Carefully remove leaves and cut thick ribs so they will lie flat. Stack 2 - 3 cabbage leaves and place about 1/4 cup of meat mixture on the center of leaves. Roll up leaves and tuck the ends in toward the center. Use wooden picks or skewers to fasten leaves securely or tie with string. Melt butter in heavy skillet and brown cabbage lightly on all sides. Add stock as needed. Add caraway seeds. Simmer for 45 minutes, adding tomatoes. If desired, thicken gravy with a flour and cold water paste. Season to taste.