Place two layers of paper towels and set the thawed spinach in the middle. Wrap the towels around it and press firmly to get rid of extra liquid. Unwrap the spinach.
In a medium bowl, combine the spinach, feta, cream cheese, and garlic, mixing thoroughly.
Slice each chicken breast lengthwise without cutting all the way through, leaving about half an inch connected at the ends to form pockets.
Spoon the spinach mixture evenly into the pockets of each chicken breast. Season the outsides with salt and pepper.
Lightly spray the air fryer basket with oil and arrange the stuffed chicken breasts inside. Cook at 350°F for 20 to 25 minutes, or until the internal temperature reaches 165°F.
Take the chicken out of the air fryer and serve it hot alongside your chosen sides.