Layer 2 paper towels and then place the thawed spinach in the center of the towels. Wrap them around the spinach and squeeze to remove as much excess moisture as possible. Unwrap the spinach.
To a medium mixing bowl add the spinach, feta, cream cheese, and garlic. Stir until well combined.
Cut each chicken breast lengthwise to create a "pocket", leaving ½-inch attached on each end. (You don’t want to cut the whole way through the breast.)
Divide the spinach mixture between the chicken breasts, stuffing it into the pockets. It's okay if the chicken pocket won't close around the filling. Season the outside of the chicken with salt and pepper.
Spray the basket of your air fryer with spray oil and place the chicken breasts in the basket. Cook at 350 degrees F for 20-25 minutes, or until the chicken is cooked through (internal temperature 165 degrees F).
Remove from the air fryer and serve warm with your favorite sides.
Notes
You can substitute 4 smaller chicken breasts instead of two large ones. Depending on the size of your air fryer, you may need to cook them in two batches.
Store any leftover chicken in an airtight container in the fridge for 3-4 days.