Place the beef in the freezer 15 minutes before ready to begin. This will help firm it up for slicing.
Slice the beef against the grain in thin strips, about ¼ inch thick.
Add the cornstarch to a large ziploc bag or bowl. Place the meat in the bag/bowl and shake or stir, until well coated. Pour out into a strainer or colander and shake off the excess cornstarch.
Heat the air fryer to 400 degrees.
Add the steak to the air fryer basket and air fry for 5 minutes, shake the basket or flip the beef halfway through cooking, and continue cooking an additional 5 minutes.
While the steak is cooking, heat the olive oil in a large deep skillet or wok on the stove. Add the garlic and ginger and cook in the oil for 30 seconds. Add the soy sauce, water, and brown sugar to the skillet and stir well.
Bring to a boil and cook for 6-7 minutes, stirring occasionally, until sauce has thickened.
Carefully add the cooked beef to the sauce and continue cooking for 1-2 minutes, tossing the beef in the sauce.
Sprinkle chopped green onions and sesame seeds over the top before serving. Enjoy!