Freeze flank steak for 15-20 minutes, until just starting to firm. Cut against the grain into 1/8-inch thick slices, cutting long slices in half with the grain into two pieces.
In a bowl, combine low-sodium chicken broth, oyster sauce, light and dark soy sauces, sesame oil, brown sugar, cornstarch, and the white and light green parts of the scallions (about 1/2 cup scallions).