Place the cut flank steak in a basin and coat with cornstarch. Let rest for 10 minutes to marinate.

Heat 2 tablespoons vegetable oil in a large pan or wok over medium-high heat. Add the steak and sauté until it is uniformly browned. Remove the steak from the pan and put aside.

In the same skillet, combine the soy sauce, water, dark brown sugar, and 2 tablespoons vegetable oil. Bring to a boil and whisk until the sugar dissolves.

Add the ginger, garlic, and red pepper flakes to the sauce and cook for about 2 minutes.

Return the steak to the pan and mix with the sauce to coat. Cook for a further 3 minutes, or until the meat is well heated.

Garnish with green onions before serving.