Start by making the sauce. Take a medium saucepan and heat 2 tablespoons of vegetable oil. Be careful not to get the oil too hot. Add garlic and ginger and immediately add soy sauce and water. Put the brown sugar in and dissolve it. Then turn the heat to a medium and let the sauce thicken. It will take around 2-3 minutes. Then remove the pan from the heat.
Cut the steak in ¼ thick pieces. Dip it in cornstarch so that a very thin coating is formed on both sides. Put the beef on a plate for 10 minutes so that the cornstarch sticks. Meanwhile, take a wok and heat one cup of oil in it, turn the stove on medium heat. The oil should be hot but not smoking.
Sauté the beef for 2 minutes in the oil. The beef will begin to get dark. Stir it a little in the wok so that it darkens evenly from all sides. After cooking for around 2 minutes, lift the meat out and put it on a paper towel so that it absorbs the oil. Take the oil out of the skillet. Now put the meat back in and let it simmer for one minute more.
Add the sauce to the meat and cook for another minute, stirring continuously. Also add the onions and cook for one more minute. With the help of a slotted spoon, take the onions and the beef out. Put them on a serving plate and serve immediately. If there is some sauce left, do not pour it on the beef.