In a medium bowl, stir together the mayonnaise, shredded Parmesan cheese, chopped onion, Dijon mustard, and garlic powder until you have a thick, pale, chunky mixture. It should be spreadable but not runny. Taste and add the optional salt and pepper if desired.
Spoon the cheesy mayo mixture evenly over the tops of the salmon fillets, pressing and spreading it so each piece is completely covered in a thick layer. The fillets should look heavily coated from edge to edge.
Place the basket in the air fryer and cook at 380°F (193°C) for 10–14 minutes, depending on the thickness of your salmon. Start checking around 10 minutes. The topping should be set, lightly golden in spots, and the salmon should flake easily with a fork.
If you prefer a deeper golden crust, air fry for an additional 1–2 minutes, watching closely so the topping doesn’t burn.
