In a large ziplock bag add the sliced flank steak and cornstarch. Toss the beef to coat evenly. Heat a large skillet to high heat and add the vegetable oil. Once heated, add the steak in a single layer and cook on each side for about a minute until the edges just start to brown. Once the steak is cooked, remove and set aside on a plate.
In a small mixing bowl combine soy sauce, brown sugar, water, ginger, and garlic. Add the sauce to the pan and bring to a boil. Add the steak to the sauce and allow the sauce to thicken for a couple of minutes. Toss with the chopped green onions and sprinkle with red pepper flakes.
