Slice flank steak or sirloin into 1/4-inch thick strips and toss with cornstarch, salt, and pepper; let sit for 10 minutes.

Heat a wok or large pan with vegetable oil to cover the bottom; fry beef in batches for 3-4 minutes per side until golden brown.

In the same pan, whisk together soy sauce, brown sugar, rice vinegar, chili flakes, sesame oil, and any remaining cornstarch; cook until thick.

Return cooked beef to sauce, toss to coat evenly, and cook for another 1-2 minutes until sticky.

Serve over warm cooked rice garnished with green onions and sesame seeds, or for a lighter dish, serve over roasted cauliflower.