In a large skillet melt margarine. Add steaks and cook over medium-high heat for 2 to 3 minutes on each side or till done. Remove from skillet, reserving drippings. Cook onion in drippings till tender. Combine broth, cornstarch, Worcestershire sauce, garlic powder and a dash pepper. Add to skillet. Cook and stir till bubbly. Cook and stir 2 minutes more. Place steaks on bread. Top with cheese and onion mixture.