Prepare the Beef:
Place the thinly sliced beef in a bowl and toss with cornstarch until evenly coated. Let it sit for 10 minutes.

Cook the Beef:
Heat 1 tablespoon of vegetable oil in a large pan over medium-high heat. Add half of the beef slices in a single layer and cook for 1-2 minutes per side until crispy. Remove and repeat with the remaining beef, adding more oil if needed. Set aside.

Make the Sauce:
In the same pan, add the minced garlic and ginger. Sauté for 30 seconds until fragrant. Pour in soy sauce, water, brown sugar, and red pepper flakes. Stir and bring to a simmer.

Combine:
Return the cooked beef to the pan and toss to coat in the sauce. Let it simmer for 2-3 minutes until the sauce thickens slightly.

Serve:
Garnish with sliced green onions and sesame seeds. Serve hot over steamed rice or noodles.