Preheat the air fryer to 375°F for 5 minutes while you prepare the filling and chicken.
Cook spinach: Heat 1 tablespoon olive oil in an 8-inch skillet over medium; add garlic and spinach and cook until wilted, 1–2 minutes, then squeeze dry and cool.
Mix filling: In a bowl, combine softened cream cheese, shredded mozzarella, grated Parmesan, Italian seasoning, cooled spinach, salt, and pepper until homogenous.
Prepare breasts: Butterfly each chicken breast by slicing horizontally leaving a ½-inch hinge, then gently pound to even thickness about ½-inch; season inside and out with ½ teaspoon salt and ½ teaspoon pepper total.
Stuff and seal: Divide filling into four ¾-cup portions, place inside each pocket, press closed, and secure seam with one toothpick; optionally brush outside with 1 tablespoon olive oil and press 1 tablespoon panko on top for crust.
Chill: Refrigerate assembled breasts on a plate for 10 minutes to set the filling and reduce leaking.
Air-fry: Place breasts seam-side down in a single layer in the preheated basket; cook at 375°F for 9–11 minutes, flip and cook 9–11 more minutes until internal temp reaches 165°F and exterior is lightly browned.
Rest and serve: Transfer to a cutting board, rest 5 minutes, remove toothpicks, slice to reveal filling and serve with chosen sides or sauce.
