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 ¼ 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 saucepan add the 2 tablespoons 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 ½ cup 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 the skillet and pour sauce over the beef.
Stir until coated and cook for a couple more minutes.
Serve over rice and garnish with green onions.