Score the beef on each side with intersecting cuts ¾ inch apart to create a diamond pattern; cut about ⅛ inch deep. Slice the scallions diagonally into ¾-inch lengths.
In a baking dish or shallow pan, combine the brown sugar, soy sauce, Stubb’s Beef Marinade, garlic, ginger, sesame oil and seeds, pepper flakes, and sherry. Stir in the scallions, then add the beef, coating well with the marinade. Cover and marinate, refrigerated, preferably for 8 hours, turning the beef once or twice.
Prepare a grill for direct cooking.
Grill the steak over direct high heat, flipping once, long enough to lightly char the scored edges of the meat, making them crisp and crunchy, without overcooking the interior. (Allow 8 to 10 minutes total for medium-rare.)
Let the steak rest for 10 minutes. If desired, boil the marinade for 1 full minute or more, to use as a sauce. Slice the steak against the grain into thick strips and serve drizzled with the cooked marinade.