Trim fat from steak. Cut steak into 8 serving-size pieces. Brown meat, half at a time, in hot oil in a large skillet. Drain off fat. Place onions in a 3-1/2 or 4-quart slow cooker. Place steak pieces and mushrooms on onions. Combine beef gravy and mushroom gravy mix; pour over gravy mixture over all.
Cover; cook on low-heat setting for 8 to 10 hours or on high-heat setting for 4 to 5 hours. Serve over mashed potatoes, if desired. Makes 8 servings.