In a small bowl, whisk together the egg and milk.
In a medium bowl, combine breadcrumbs with garlic powder and onion powder, then season with salt and pepper. Whisk together with a fork to fully combine.
Prepare the chicken patties one at a time. First, fully coat one patty in the egg wash and let excess liquid drip off before transferring it to the breadcrumbs. Evenly coat the entire patty with breadcrumbs, pressing the breadcrumbs into the chicken well so they stick, then shake off any excess.* Repeat this process to double coat each patty (egg, breadcrumbs, egg, breadcrumbs) and set aside.
Preheat your air fryer to 360 degrees.
Transfer the patties to the air fryer basket and cook for 18 minutes, flipping halfway through.
Check to make sure the patties are slightly crispy on both sides - this is how you will know if they’re done. (Alternatively, you can check the temperature with a meat thermometer). If they’re ready, top each patty with a slice of cheese and put them back in the air fryer, but don't turn it on! The heat of the air fryer will quickly melt the cheese in about 1 to 2 minutes.*
Spread the mashed avocado on the bottom half of both buns.
Once the cheese is melted, carefully remove patties from the basket and place on buns. Top each sandwich with a slice of tomato and a handful of baby arugula. Serve immediately.