Prepare the Beef: In a bowl, toss the sliced flank steak with cornstarch until evenly coated. Let it sit for 10 minutes.
Heat the Oil: In a large skillet or wok, heat vegetable oil over medium-high heat.
Cook the Beef: Add the coated beef in batches, frying for about 2-3 minutes per side until crispy. Remove and set aside.
Make the Sauce: In the same pan, add garlic and ginger, sautéing for 30 seconds. Then add soy sauce, water, brown sugar, and red pepper flakes. Bring to a simmer.
Thicken & Combine: Let the sauce cook for 3-4 minutes until slightly thickened. Add the crispy beef and toss to coat evenly.
Finish & Serve: Drizzle with sesame oil, then garnish with green onions and sesame seeds. Serve immediately with rice or noodles.