Place the steak in a bowl and cover with the cornstarch, removing any excess.
Preheat the air fryer for 5 minutes at 375°F (190°C).
Spray the air fryer basket with some vegetable oil.
Place the beef steak in the air fryer basket for 4 minutes*, shaking and rotating them every 2 minutes or so that the juices and oils cover the beef.
In a small sauce pan at medium heat, cook the ingredients for the sauce for about 5 minutes (except the sesame seeds and spring onions). Mix the sauce throughly.
Open the air fryer to see if they are cooked enough for you, and heat them for a couple more minutes if you wish.
Place the steak in a bowl and cover with the sauce.
Sprinkle with sesame seeds and spring onions.
Serve while warm.
Notes
* Adjust the timing of the air fryer depending on how you want your steak cooked.