Place the beef in the freezer 15 minutes before ready to begin. This will help firm it up for slicing.
Slice the beef against the grain in thin, bite-sized strips, about 1/4 inch thick.
Add the cornstarch to a Ziploc bag. Place the meat in the bag and shake, until well coated. Pour out into a strainer or colander and shake off the excess cornstarch.
In a small sauce pan add the oil, soy sauce, brown sugar, garlic, ginger, water, rice wine vinegar and red pepper flakes. Stir and cook over medium-high heat until sauce thickens (15-20 minutes). Set aside.
Meanwhile, in a large skillet heat the oil over medium-high heat. When the oil is hot, add some of the beef in a single layer and cook until browned. You will need to do this in batches. Set aside the browned beef on a plate lined with a paper towel and tented with foil.
Return all beef to skillet and pour sauce over the beef. Stir until coated and cook for a couple more minutes.