Marinate the sliced flank steak in soy sauce and brown sugar for 15 minutes.
Sprinkle cornstarch over the marinated beef and toss to coat evenly.
Heat vegetable oil in a large pan over medium-high heat.
Add the marinated beef to the pan in a single layer, cooking until browned (about 3-4 minutes).
Remove the beef from the pan and set aside.
In the same pan, add minced garlic and ginger, sautéing for 30 seconds.
Return the beef to the pan and pour in any remaining marinade.
Add red pepper flakes if using, and stir to combine.
Cook for an additional 2-3 minutes until the sauce thickens.
Serve hot, garnished with chopped green onions and sesame seeds.