Begin by marinating the sliced flank steak. In a bowl, combine the soy sauce, brown sugar, minced garlic, grated ginger, and red pepper flakes (if using). Add the sliced beef and let it marinate for 10 minutes.
While the beef is marinating, heat the vegetable oil in a wok or large frying pan over medium-high heat.
Once the oil is shimmering, add the marinated beef to the pan in a single layer. Allow it to cook without stirring for about 2-3 minutes, searing the meat until it's browned.
Stir the beef gently, then cook for an additional 2-3 minutes until cooked through.
Sprinkle the cornstarch over the beef and mix well, allowing it to thicken the sauce. If you like, you can add a little water to reach your desired sauce consistency.
Add the scallions to the pan and cook for 1-2 minutes until they are tender but still crisp.
Taste and adjust seasoning with salt and pepper as necessary.
Serve hot over steamed rice or noodles for a complete meal.