In large skillet, over high heat, cook steaks in margarine 5 minutes on each side for to desired doneness. Remove from skillet, keep warm.
In same skillet, sauté mushrooms and onion until tender. Stir in flour until well blended. Add milk; cook until mixture thickens and begins to boil, stirring constantly. Sir in steak sauce. Serve sauce over steaks and rice if desired.