Cook the rice in two cups of water. Bring rice and water to a boil, cover, reduce heat and let it cook until all the water has been absorbed and rice is tender.
Mix ground beef, rice, onion, margarine, and egg. Season with salt and pepper to taste.
Grease a 3 quart casserole dish. Line bottom of casserole with 1/2 cabbage; add 1/2 meat mixture; top with rest of cabbage, then rest of meat mixture. Top with chopped tomatoes or tomato sauce.
Cover, bake at 350 degrees F / 175 degrees C for 1 hour; uncover and bake an additional 30 minutes.