Clean the steak and thinly cut against the grain. Tip: place in a freezer for a few minutes this helps with the slicing. Coat with cornstarch and set aside.
In a bowl, whisk the remaining ingredients except for oil and set aside.
Heat the oil in a skillet and add the beef in a single layer (make sure you shake off the excess cornstarch), cook on both sides for about 2 minutes (both sides). Remove and set aside.
Add the sauce to the same and bring to a boil and allow to thicken a little bit and use a wooden spoon to scrape the bottom of the skillet. Add the cooked steak and combine to allow the sauce to coat the beef. In case the sauce is not thickened to your preference make a slurry (cornstarch and water) and add a small amount of the slurry to the skillet. Allow everything to warm up and well coated.
Serve and garnish with green onion. You can double the sauce if you desire.
Enjoy with rice or as is.