Toss the sliced flank steak with 1/4 cup cornstarch until every piece is evenly coated.
Preheat the air fryer to 400°F (200°C). Spray the basket with cooking spray, arrange the beef in a single layer, and spray the tops.
Air fry for 10 to 12 minutes, shaking the basket halfway through, until the beef is crispy and browned on the edges.
While the beef cooks, combine soy sauce, brown sugar, garlic, ginger, and red pepper flakes in a small saucepan over medium heat. Bring to a simmer, stir in the cornstarch slurry, and cook for 1 to 2 minutes until the sauce is thick and glossy.
Toss the crispy beef and green onion pieces together in the sauce until fully coated. Garnish with sesame seeds and serve over steamed rice.