While the beef is still frozen, cut the steak with the grain at a 30° angle to the cutting board into thin, wide slices. Ideally, let beef slices defrost in the refrigerator for an hour. Lay out some plastic wrap on a cutting board and put three or four slices on top, separated from each other. Cover with more plastic wrap and pound to ¹/₈" thickness. Lay three slices, overlapping each other by a quarter inch vertically, in front of you. Sprinkle with salt and pepper and sesame seeds. Lay one scallion across the beef, a half-inch from the edge closest to you. Using the plastic wrap to help you, tightly roll up the beef like a jellyroll. Repeat with the rest of the meat.

Place rolls in a wide dish. Pour all but a scant quarter-cup of cooled marinade over the beef rolls. Marinate beef for at least 30 minutes, preferably a few hours. Cook the rest of the marinade until it reduces to a semi-thick syrup, and reserve.

Heat a grill or skillet over high heat. Add the oil and when the grill or pan is very hot, sear the beef rolls, first on their open edge to seal, until well-browned. Flip them over and sear the other side. The whole cooking process should be very fast, at the most 30 seconds per side. You want the rolls well-browned on the outside but still medium-rare on the inside.

Let the rolls rest for a minute. Slice them into bite-sized pieces, much like sushi. Stand the pieces on their ends and brush a bit more sauce on the top. Eat immediately; they are best very hot.

Substitutions: Asparagus stalks or fresh bamboo shoots may be used in place of the scallions.