Preheat the air fryer to 400°F for 3 minutes.
Wash, dry, and trim the squash. Slice into even rounds between 1/4 and 1/2 inch thick. Pat the slices dry.
Add the flour to a plate. In a shallow bowl, whisk the milk and cornstarch until smooth. On a third plate, combine the cornmeal, Parmesan, and garlic salt.
Dredge each slice in flour, then the milk mixture, then the cornmeal. Press the cornmeal on firmly with your fingers so it adheres.
Grease the air fryer basket and arrange the squash in a single layer, leaving space between pieces. Do not overcrowd.
Spray the tops with cooking spray. Air fry at 400°F for 8 to 10 minutes, flipping and re-spraying at the halfway point.
Cook until deep golden and crisp. Transfer to a wire rack, repeat with remaining squash, and serve immediately.