Rinse beef and pat dry. Cut steak at a 45° angle across the grain, cutting almost, but not completely, through. Make another cut 1/4 inch from the first cut, following the same angle and cutting all the way through. Open up butterflied slices and place in a large bowl.
Add soy, green onions, sesame oil, sugar, and garlic to bowl; mix to coat. Cover and chill at least 30 minutes or up to 4 hours.
Spread slices open on an oiled grill over a bed of hot coals or high heat on a gas grill (you can hold your hand at grill level only 2 to 3 seconds); close lid on gas grill. Cook, turning once, until browned on both sides, 5 to 6 minutes total.