Place mozzarella between bread slices, making 2 sandwiches. Cut each sandwich in half diagonally.
Beat egg in shallow dish. Mix together flour and ¼ teaspoon salt in another dish. Mix together panko and parmesan cheese in a third dish. Dip sandwich halves, one at a time, into the flour mixture to coat on all sides. Shake off excess flour. Dip in egg and then into panko mixture, pressing panko to stick.
Preheat air fryer to 390 degrees, spray basket with olive oil. Spray sandwiches on all sides. Arrange in basket and air fry for 5 minutes or until crisp and browned, turning once with tongs.
Cut each sandwich in half making 8 triangles. Serve immediately.
Notes
For even better results, toss the panko with ½ teaspoon oil and microwave for 20 seconds. This will give the panko a head start and create a beautiful toasted color.