Toss sliced flank steak with cornstarch until evenly coated.

Heat oil in a skillet and cook beef in batches until browned and crispy, then set aside.

In the same skillet, sauté garlic and ginger, then add soy sauce, water, and brown sugar and simmer until slightly thickened.

Return beef to skillet, add green onions and sesame oil, and toss until sauce thickens and coats the beef.

Serve hot over rice or noodles and garnish with sesame seeds if desired.