If not yet sliced, thinly slice your cut of meat so they are bite-sized pieces.
Marinade the beef in soy sauce, Shaoxing wine, and cornstarch. Toss to evenly coat and let sit for at least 15 minutes.
Heat up oil in a pan on medium heat and stir-fry beef until brown, but not fully cooked. Remove from pan and set aside in a separate bowl.
Cook down the onions in the same pan on medium heat. Be patient as we want the onions to get soft so this could take up to 10 minutes.
When onions are soft, add the cooked beef back in the pan. Add in the sauce mixture. Continue to stir-fry until beef is completely cooked, about another 3-4 minutes.
Add in the chopped scallions and give it one last toss. Serve with a bowl of rice.