Use a large frying pan or a wok and set it on the stove on high heat.
Once the pan is very hot, add in the oil and sliced beef. Don't be shy with the oil, it will help the beef not stick to the pan and sear it better. I use approximately 3-4 tablespoons of oil.
Spread the beef out throughout the surface of the hot pan to brown it to create an initial sear. Then stir fry the beef until it is about 90% cooked, approx. 2-4 minutes depending on how thinly you sliced it. Don't worry it will cook the rest of the way in the sauce at the end. Remove it from the pan and set it aside for later.
Add a bit more oil into the pan, and the ginger slices. Toast the ginger for 1 minute until it is slightly brown.
Then add in the onion slices and ginger and stir fry it until it is soft and slightly brown. Approx. 2 minutes.
Push the onions, ginger and garlic to the side of the pan.
Mix the stir-fry sauce again (cornstarch will settle to the bottom), and pour the sauce into the hot pan. Keep stirring the sauce in the pan until the sauce has turned thick and it has turned from a milky color to a dark brown color.
Once the sauce is thick, mix the onions into the sauce, and add the beef back in as well. Add the green onions as well and stir fry everything for about 1 minute.
Plate and enjoy with steamed rice!