Trim the flank steak of any silver skin or excess fat.
Whisk the oil, soy sauce, lime zest, lime juice, garlic cloves, mustard powder, salt, and pepper together in a small bowl. Rub the marinade all over the flank steak. Cover the steak or place in a resealable plastic bag and marinate in the refrigerator for at least 1 hour or overnight.
Remove the flank steak from the refrigerator 30 minutes before cooking to break the chill.
Select the Preheat function on the COSORI Air Fryer, adjust the temperature to the 400°F then press Start/Pause.
Set the temperature to 400°F and time to 14 minutes, then press Start/Pause.
Flip the steak over.
Set the temperature to 400°F and time to 12 minutes, then press Start/Pause.
Remove the steak and let it rest for 10 minutes before slicing it against the grain and serving, seasoned with additional salt.