Combine sauce ingredients in a small mixing bowl, whisk and set aside.

Add oil to a large heavy skillet (or wok if you have one) and heat over HIGH heat until oil is very hot and smoking slightly.

Add onion and garlic to the skillet and saute for 1 minute, careful not to let the garlic burn.

Next, add the sliced flank steak and stir fry for about 1 minute. Slide beef, onion and garlic onto a plate and set aside.

Reduce the heat to MED/MED HIGH and add the whisked sauce to the skillet.

Cook for 1 minute, until sauce becomes thicker and syrupy.

Add the beef, onion, and garlic back to the skillet (make sure to get all the juices that were on the plate too, that's flavor!).

Heat until warmed through and coated in the sauce.

Be sure not overcook the beef - you want tender pieces of steak, not chewy ones!

Serve sprinkled with sesame seeds and chopped chives/scallions if desired.