In a large bowl, toss the sliced flank steak with cornstarch until evenly coated. Let it sit for about 15 minutes to allow the cornstarch to adhere to the meat.

Heat the vegetable oil in a large skillet or wok over medium-high heat. Once hot, add the beef in a single layer (you may need to do this in batches) and cook until browned, about 2-3 minutes per side. Remove the beef and set aside.

In the same skillet, add the white parts of the green onions, garlic, and ginger. Sauté for about 1 minute until fragrant.

In a small bowl, whisk together the soy sauce, brown sugar, water, sesame oil, and red pepper flakes (if using). Pour this mixture into the skillet and bring to a simmer.

Return the cooked beef to the skillet, stirring to coat it in the sauce. Cook for an additional 2-3 minutes until the sauce thickens slightly.

Stir in the green parts of the green onions and remove from heat. Serve hot over steamed rice or noodles.