Preheat air fryer to 375°. Cut a pocket horizontally in the thickest part of each chicken breast. Fill with onion and green pepper. In a small bowl, combine olive oil and seasonings; rub over chicken., In batches, place chicken on greased tray in air-fryer basket. Cook 6 minutes. Top chicken with cheese slices; secure with toothpicks. Cook until a thermometer inserted in chicken reads at least 165°, 6-8 minutes longer. Discard toothpicks. If desired, serve with toppings of your choice.