Heat a large frying pan or wok on high heat.
Once hot, add oil and the sliced beef. Spread the beef to brown and create an initial sear. Stir fry until about 90% cooked (approx. 2-4 minutes). Remove and set aside.
Add more oil to the pan, then add ginger slices. Toast until slightly brown (about 1 minute).
Add onion slices and garlic. Stir fry until soft and slightly brown (approx. 2 minutes).
Push the onions, ginger, and garlic to the side of the pan.
Stir the sauce again (cornstarch will settle), and pour into the pan. Stir until the sauce thickens and turns dark brown.
Mix onions into the sauce, add the beef back in, and stir in green onions. Stir fry for about 1 minute.
Plate and serve with steamed rice.
Enjoy your delicious homemade Chinese Beef and Onion Stir Fry!