Directions

1. Wrap steak in plastic wrap, and place in the freezer for 30 minutes. Unwrap and slice across the grain into 1/4-inch thick slices.
2. In a large bowl, combine flank steak and cornstarch.
3. Heat vegetable oil in a large cast iron skillet. Working in batches, add beef and fry until browned and cooked through, about 1-2 minutes. Transfer to a paper towel-lined plate; discard excess oil.
4. Add beef and soy sauce mixture to the skillet over medium heat and cook, stirring occasionally, until sauce thickens, about 2-3 minutes. Stir in green onions.
5. Serve immediately, garnished with sesame seeds, if desired.
6. In a small saucepan over medium heat, combine water, brown sugar, soy sauce, garlic, ginger and vegetable oil. Bring to a boil; reduce heat and simmer, stirring occasionally, until slightly thickened, about 5-10 minutes.