Preheat the Air Fryer to 390°F for 3 minutes.
Squeeze the thawed spinach completely dry using paper towels or a clean kitchen towel.
In a bowl, mix ricotta, spinach, harissa paste, black pepper, and salt until well combined.
Pat the salmon fillets dry with paper towels.
Cut a horizontal pocket into the side of each fillet, about 2 inches deep, without cutting through.
Stuff each pocket with half of the ricotta mixture, pressing gently to pack it in.
Place the stuffed salmon in the Air Fryer basket, stuffed side up, leaving space between fillets.
Cook at 390°F for 12 minutes without opening the basket.
Check the internal temperature with an instant read thermometer; it should read 125°F in the thickest part.
Let rest for 2 minutes before serving.