Cut the flank steak lengthwise (with the grain) into strips 2 in. (5 cm) wide. Then cut the strips crosswise into very thin slices. (This is easier if the meat is partially frozen.)
Toss the meat with the soy sauce, sherry, and cornstarch. Let marinate 30 minutes or longer.
Stir-fry the beef in 2 or more batches, depending on the size of the pan or wo