- 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.