Heat 2 teaspoons vegetable oil in a small saucepot over medium-low heat. Add the ginger and garlic and saut� until golden, about 2 minutes. Add the soy sauce and water, stirring to combine.
Stir in the brown sugar and increase the heat to medium. Bring the sauce to a boil for 3 minutes. Remove the sauce from the heat and set aside.
Slice the flank steak against the grain into 1/4-inch pieces, then toss it with the cornstarch. Place the coated pieces of steak in a sieve and shake off any excess cornstarch. Allow the steak to sit 10 for minutes.
Place a large saut� pan or wok over medium-high heat and add 1/2 cup vegetable oil. Once the oil is hot (but not smoking), add the beef to the pan and saut� for 2 minutes until it is seared on all sides but barely cooked in the center. Remove the steak from the pan with a slotted spoon and transfer it onto a paper towel-lined plate. Pour any excess oil out of the wok.
Place the saut� pan back over medium heat. Add the prepared sauce to the hot pan (it should come to a boil almost immediately), then add the reserved steak and cook at a boil, stirring constantly, 2 minutes. Add the sliced scallions, stirring to combine.
Transfer steak and scallions with a slotted spoon to a plate and serve.
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