1. Combine teriyaki sauce, garlic and pepper in a large ziplock bag. Add steak, seal and turn several times to coat with marinade. Let stand 15 minutes at room temperature.
2. Preheat grill to high. Thread meat onto thin metal skewers; oil grates. Grill steak, turning once, until cooked to desired doneness (1 to 2 minutes per side for medium-rare). Transfer skewers to a serving platter, sprinkle with scallions and sesame seeds and serve.