Prepare the Beef. In a large bowl, combine the beef slices with cornstarch, salt, and pepper. Toss to coat the beef evenly.
Fry the Beef. Heat oil in a large skillet or wok over medium-high heat. Fry the coated beef slices in batches until they are golden brown and crispy. Remove and drain on paper towels.
Make the Sauce. In the same skillet, add a little oil if needed. Sauté the minced garlic, ginger, and red chili flakes until fragrant. Add the soy sauce, brown sugar, rice vinegar, and sesame oil. Stir to combine and cook until the sauce thickens and becomes sticky, about 3-5 minutes.
Combine and Serve. Add the fried beef slices to the sauce and toss to coat evenly. Cook for an additional 2 minutes until the beef is heated through and well-glazed.