In a large bowl, toss the sliced flank steak with cornstarch until evenly coated. Let it sit for about 10 minutes to absorb the cornstarch.
Heat the vegetable oil in a large skillet or wok over medium-high heat. Add the beef in a single layer and cook for about 2-3 minutes on each side until browned. You may need to do this in batches to avoid overcrowding the pan. Remove the beef and set aside.
In the same skillet, add the soy sauce, brown sugar, garlic, ginger, and red pepper flakes. Stir well and bring to a simmer. Let it cook for about 2-3 minutes until slightly thickened.
Return the beef to the skillet and toss to coat it in the sauce. Cook for an additional 2-3 minutes until heated through.
Stir in the chopped green onions just before serving.
Serve hot over steamed rice or noodles for a complete meal.