Sprinkle both sides of steaks with salt and pepper. Let stand at room temperature 20 minutes.
Pat steaks dry with a paper towel. Place steaks on grill rack coated with cooking spray; grill 3 minutes on each side or until desired degree of doneness. Grill buns, cut sides down, 2 minutes or until lightly browned. Remove steak and bread from grill. Cover steak loosely with foil; let stand 5 minutes. Spread 1 1/2 teaspoons mustard over cut sides of buns. Place steaks on bottom halves of buns. Top each serving with 1 lettuce leaf, 1 tomato slice, 1 onion slice, and top half of bun.