In a bowl, combine the sliced flank steak, soy sauce, cornstarch, and minced garlic. Toss until the beef is well-coated. Let it marinate for about 15-20 minutes.

Lightly brush or spray the air fryer basket with vegetable oil to prevent sticking. Place the marinated beef in a single layer in the air fryer basket. Cook for about 8-10 minutes, flipping the beef halfway through, until it is browned and crispy. The cooking time may vary depending on the thickness of the beef and the air fryer model you're using. Cook in batches if necessary.

While the beef is cooking, prepare the sauce. In a small saucepan, combine the hoisin sauce, brown sugar, rice vinegar, freshly grated ginger, and red pepper flakes (if using). Stir well to combine and heat the sauce over low heat until the sugar has dissolved and the sauce is warmed through.

Once the beef is cooked, transfer it to a large bowl and pour the sauce over it. Toss the beef in the sauce until it is well-coated.

Garnish the Mongolian beef with sliced green onions and sesame seeds (if desired) for added freshness and texture.

Serve the Air Fryer Copycat PF Chang's Mongolian Beef hot over steamed rice or noodles.