In medium bowl, combine ground beef, onion, mushrooms, garlic powder, salt, pepper and Worcestershire sauce; mix well. Shape mixture into 4 oval-shaped patties, about 1/2 inch thick.
Heat large nonstick skillet over medium heat until hot. Add patties; cook 10 to 12 minutes or until thoroughly cooked, turning once. Remove patties from skillet; cover to keep warm. Drain drippings from skillet and discard; carefully wipe skillet with paper towels.
To assemble sandwiches, spread margarine on one side of each bread slice. With margarine on outside of sandwiches, assemble 4 sandwiches with cheese slices and ground beef patties.
Heat same skillet over medium heat until hot. Add sandwiches; cook about 1 minute on each side or until bread begins to brown and sandwich is hot. If desired, serve with ketchup, mustard or chili sauce.