Heat gas or charcoal grill. Cut 18-inch square sheet of heavy-duty foil. Brush onion wedges with oil. In small bowl, combine 1 teaspoon of the garlic-pepper blend and the dry mustard; mix well. Sprinkle both sides of steak with mustard mixture.
When grill is heated, place steak on gas grill over medium heat or on charcoal grill 4 to 6 inches from medium coals. Place onion wedges on sheet of foil next to steak. Cook 8 to 12 minutes or until steak is of desired doneness and onion is crisp-tender, turning steak once and stirring onion occasionally.
Meanwhile, in small bowl, combine butter and remaining 1/2 teaspoon garlic-pepper blend; blend well. Spread on cut sides of buns. To toast buns, place cut sides down on grill during last 1 to 2 minutes of cooking time.
To serve, spread cut sides of each bun with about 1 teaspoon steak sauce. Cut steak crosswise into thin slices. Layer steak slices, onion wedges and tomato slices on bottom halves of buns. Cover with top halves of buns.