Prepare the Beef: Toss thinly sliced flank steak or sirloin with cornstarch, salt, and pepper in a bowl. Let it sit for about 10 minutes.

Cook the Beef: Heat a large skillet over medium-high heat with vegetable oil. Add the beef in a single layer and cook for 2-3 minutes until golden and crispy.

Make the Sauce: In a bowl, combine soy sauce, rice vinegar, hoisin sauce, brown sugar, tomato paste, chilli flakes, minced garlic, and minced ginger. Whisk until blended.

Cook Sauce: Pour the sauce into the skillet, scraping any brown bits. Simmer for 1-2 minutes until the sauce thickens slightly.

Combine and Serve: Return the crispy beef to the skillet and toss in the sauce. Garnish with sliced green onions and sesame seeds, serving over rice or noodles.