In a bowl, toss the sliced flank steak with cornstarch until evenly coated. Let it sit for 10 minutes.

Heat vegetable oil in a large skillet over medium-high heat. Add the beef slices in a single layer and cook until crispy and browned, about 2-3 minutes per side. Remove and set aside.

In the same skillet, add minced garlic and grated ginger; sauté until fragrant, about 30 seconds.

Pour in soy sauce, water, and brown sugar. Stir until the sugar dissolves. Add red pepper flakes if desired.

Bring the sauce to a boil, then reduce heat to simmer. Return the cooked beef to the skillet and toss to coat with the sauce.

Add green onions and cook for another 1-2 minutes until the sauce thickens to your liking.

Garnish with sesame seeds before serving.