Preheat your air fryer for a couple minutes at 360 F.  For cooking frozen salmon, start it without any seasoning.  This is basically a "thawing" step.  I usually cook it at 350-360 F for 5-7 minutes for a center-cut filet that's 1 to 1 1/2 inch thick (thinner pieces will need less).

Then pause the air fryer briefly.  Sprinkle with salt, then squeeze dijon mustard on it and spread a thin layer over the top of the filet (doesn't hurt to get the sides, it's just a bit trickier and not mission-critical).  I use a silicone basting brush for this. 

Then sprinkle with a bit more salt and whatever other spice mix you want...I have several I love, including Trader Joe's Everything Bagel, 21 Seasoning Salute, and Chili Lime. 

Then cook for another 5 or so minutes, sometimes more.  I sometimes do this at the same temperature, and sometimes I up it to 375 F.  You want the outside of the fish to look opaque and flaky but you definitely want to use a thermometer.

Frozen salmon tends to cook unevenly, with the outside coming to temperature faster than the frozen middle. So, always be sure to check the temperature of the very center of the salmon, or at the very thickest part of the salmon, before serving, to make sure it’s not still cold in the center. See notes below about internal temperatures for doneness.