Pat dry steak slices. Using your hands or a spoon, mix them with the cornstarch, until all pieces are coated. Place beef slices in a strainer and shake off excess cornstarch.
Heat half of the oil in a large wok at medium-high and add the garlic and ginger. Immediately add the soy sauce, water, brown sugar and pepper flakes. Cook the sauce for about 2 minutes and transfer to a bowl. Don't worry if the sauce doesn't look thick enough at this point. The cornstarch in the beef will thicken it up later.
Turn the heat up and add the remaining oil to the wok. Add the beef and cook, stirring until it is all browned (this is a quick thing). Pour the sauce back into the wok and let it cook along with the meat. Now you can choose to cook it down and reduce the sauce or leave it thinner. Add the green onions at the last minute so the green parts will stay green and the white parts crunchy. Serve hot over rice.