Freeze the meat until firm, about 2 hours. Meanwhile, in a heavy iron skillet, spread the sesame seeds and toast over moderate heat until golden. Stir the seed once or twice to toast evenly. With a mortar and pestle or with the back of a spoon against a board, grind the seeds until powdery. Place in a large bowl with the scallions, garlic to taste (if you like garlic, up to 3 cloves may be used), soy sauce, sugar, sherry, and vegetable oil, blending well. With a very sharp knife, cut the semi-frozen meat (beef chuck, round or sirloin), across the grain, into strips about 1/4-inch thick. Add the meat strips to the sauce, stirring to coat well, cover, and marinate at room temperature for 1 to 2 hours. Let the fire in a hibachi or small grill burn down ot glowing coals. If the grill rack is widely spaced, place the meat strips in a hinged wire broiler. Broil about 1 minute on each side until browned but still rare. Serve with rice and vinegary coleslaw or cucumber salad.