In a small saucepan over medium heat, mix sesame oil, soy sauce, water, sugar, ginger, garlic, and red pepper flakes. Let simmer for 7-10 minutes, until it is ¼ reduced. Take off heat and set aside.
In a medium sized bowl, toss the beef slices in salt, pepper and cornstarch.
In a large skillet or saute pan, heat vegetable oil over medium-high heat and place steak pieces in one layer in batches. Cook for 2 minutes on one side then another minute on the other, the beef should have a brown crispy crust on each piece. Scoop each batch up onto a plate lined with paper towels to absorb excess oil.
After each batch is done, discard the excess oil from the pan. Add soy sauce mixture and beef into the pan and let the sauce reduce for 2-5 minutes (or more, you want it to thicken and reduce as a glaze but don't let it completely evaporate and burn) over medium/medium low heat.
Toss in sesame seeds and green onion slices then serve immediately with white rice.