If using packaged pre-sliced meat, separate the slices. Add the meat, onion, and scallions to the marinade. Toss gently, with hands, to mix everything well. Marinate for 30 minutes to an hour in the fridge.
Heat a skillet over medium high heat. Add the meat along with any remaining marinade. Cook the meat for about a minute. Stir-fry until the meat and vegetables are cooked.