Start by slicing the flank steak thinly across the grain. Place the sliced beef in a mixing bowl and sprinkle the cornstarch over it. Toss the beef until it is evenly coated with cornstarch.
In a large skillet or wok, heat the vegetable oil over medium-high heat until hot.
Carefully add the cornstarch-coated beef to the skillet in a single layer. Fry the beef for about 2-3 minutes on each side until golden brown. Remove the cooked beef from the skillet.
In the same skillet, add the minced garlic and ginger. Stir-fry for about 30 seconds until fragrant. Pour in the soy sauce, dark brown sugar, and water. Stir until the sugar is dissolved.
Return the cooked beef to the skillet and toss it in the sauce, cooking for an additional 2-3 minutes until the sauce thickens slightly.
Add the sliced green onions and drizzle the sesame oil over the beef. Stir everything together and cook for an extra minute.
