Prepare the Beef:
In a bowl, toss the sliced flank steak with cornstarch until evenly coated. Let it sit for 10 minutes.

Cook the Beef:
Heat vegetable oil in a large skillet or wok over medium-high heat. Sear the beef in batches for 2-3 minutes until crispy. Remove and set aside.

Make the Sauce:
In the same skillet, reduce heat to medium. Add garlic and ginger, sautéing for 30 seconds. Pour in soy sauce, water, and brown sugar. Stir well and simmer for 2 minutes.

Combine:
Return the cooked beef to the skillet and toss to coat in the sauce. Add sesame oil, red pepper flakes, and green onions. Cook for another 2 minutes.

Serve:
Garnish with sesame seeds and serve hot over rice or noodles.