Sprinkle steak with salt and pepper. Grill, covered, over medium-hot heat for 6-10 minutes on each side or until meat reaches desired doneness (for medium-rare, a meat thermometer should read 145°; medium, 160°; well-done, 170°). Let stand for 5 minutes before thinly slicing.
Spread butter over inside of buns. Place the tomatoes, onion, sliced steak and cheese on bun bottoms. Broil 5-6 in. from the heat for 2-3 minutes or until cheese is melted. In a small bowl, whisk the mayonnaise, mustard and Worcestershire sauce until blended; spoon over cheese. Replace bun tops. Yield: 6 servings.