Combine sugar, soy sauce, sake, garlic, scallions(white part only-reserve the green part), ginger and sesame oil in a medium size flat dish. Add the steak and coat well with the marinade. Let marinate at room temperature for 30 minutes and up to overnight, turning once.
Remove meat from marinade and grill on each side for 2 to 3 minutes, for medium rare. If you like, reduce remaining marinade by about half and use to drizzle over grilled meat.
Brush the remainder of the scallions(the green parts) with the oil and season with salt and pepper and grill them with the meat.
Remove from grill, cover and let rest for 5 minutes. Slice thinly against the grain.