Whisk together the first five ingredients, then add the sliced meat and let it marinate for 10 minutes.
Warm the oil in a skillet over medium-high heat. Add the steak (reserving the marinade) and cook for 5 minutes, stirring constantly.
Stir the cornstarch into the marinade, add the mixture to the skillet, and cook for another minute, continuing to stir. Sprinkle on the scallions and serve.