Start by slicing the flank steak into thin, bite-sized strips. Coat in cornstarch and let sit for about 10 minutes.
In a skillet or wok, heat vegetable oil over medium-high heat. Add beef in small batches, cooking until browned. Remove and set aside.
In the same pan, lower heat to medium and add minced garlic and grated ginger, stirring for 30 seconds. Add soy sauce, water, and brown sugar, stirring until dissolved.
Return beef to the pan, mixing well. Let simmer for 5 minutes to allow flavors to merge and sauce to thicken.
Sprinkle chopped green onions and sesame seeds over the beef. Serve hot, paired perfectly with rice.
Notes
Store leftovers in an airtight container in the fridge for up to 3 days. Reheat using a microwave or pan with a splash of water if necessary.