Slice flank steak against the grain into 1/4-inch slices.
In a large bowl, toss the steak with the cornstarch so it is coated on all sides. Allow it to sit for 10
minutes.
In a small saucepan over medium heat, mix 1 tablespoon oil, soy sauce, water, sugar, ginger, and garlic. Let simmer for 10 minutes, until it reduces just a little. Take off heat and set aside.
In a large skillet, heat remaining oil over medium-high heat and place steak pieces in one layer in batches, stir-fry for about 2 minutes per side. Scoop each batch up onto a plate lined with paper towels to absorb excess oil.
Add the soy sauce mixture to the skillet. When it starts to simmer, return the steak and stir-fry the sauce for 2 to 3 minutes. Add the scallions, and sesame seeds stir to combine, and then turn off the heat.
Serve with rice or noodles.