Toss the sliced flank steak with cornstarch in a bowl until evenly coated. Let it sit for 10 minutes while you prepare the sauce.

In a small bowl, whisk together the soy sauce, brown sugar, water, and red pepper flakes if using. Set aside.

Heat 2 tablespoons of vegetable oil in a large skillet or wok over high heat. Add half the beef in a single layer and cook for 1-2 minutes per side until browned and crispy, ensuring the beef reaches an internal temperature of 145°F (63°C). Transfer to a plate. Repeat with remaining oil and beef.

Reduce heat to medium. In the same pan, add the garlic and ginger and stir-fry for 30 seconds until fragrant.

Pour the sauce mixture into the pan and bring to a simmer. Let it bubble for 1 minute to thicken slightly. Return the beef to the pan along with the 6 stalks of green onions (cut into 2-inch pieces) and sesame oil. Toss everything together until well coated and heated through, about 1 minute.

Transfer to a serving platter and garnish with the reserved sliced green onions and sesame seeds if desired. Serve hot over steamed jasmine rice.