Place the beef in a shallow dish. Combine soy sauce, sugar, green onion, garlic, sesame seeds, sesame oil, and ground black pepper in a small bowl. Pour over beef. Cover and refrigerate for at least 1 hour or overnight.
Broiled Beef Option: Preheat oven broiler to 475F/ 250C.
Place marinated beef flanks on broiler pan and cook, turning once to cook both sides equally. Cook for 3-4 minutes on both sides for medium rare steaks.
Grilled Beef Option: Preheat an outdoor grill for high heat, and lightly oil the grate.
Quickly grill beef on hot grill until slightly charred and cooked through, 1 to 2 minutes per side.