In a large bowl, toss the sliced flank steak with cornstarch until well coated. Let it sit for about 10 minutes to absorb the cornstarch.

In a separate bowl, whisk together the orange juice, soy sauce, brown sugar, rice vinegar, garlic, ginger, and red pepper flakes (if using). Set aside.

Heat the vegetable oil in a large skillet or wok over medium-high heat. Once hot, add the coated beef in a single layer. Cook for about 2-3 minutes on each side until browned and crispy. You may need to do this in batches to avoid overcrowding the pan.

Once all the beef is cooked, return it to the skillet and pour the orange sauce over the beef. Stir to coat the beef evenly and cook for an additional 2-3 minutes until the sauce thickens slightly.

Remove from heat and garnish with sliced green onions and sesame seeds before serving.