Toss sliced beef with cornstarch until evenly coated.
Heat oil in a large skillet over medium-high heat.
Fry beef in batches until crispy and browned, then remove and set aside.
In the same pan, sauté garlic and ginger for 30 seconds.
Add soy sauce, brown sugar, and water and stir until sugar dissolves.
Simmer sauce for 3–4 minutes until slightly thickened.
Return beef to the pan and toss to coat in the sauce.
Drizzle sesame oil and sprinkle red pepper flakes if using.
Cook for 2–3 minutes until sauce thickens and coats the beef.
Garnish with sliced green onions before serving.
