Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness.
In a shallow bowl, soak the chicken in the buttermilk for at least 15 minutes to tenderize.
In a separate shallow dish, whisk together the panko, garlic powder, onion powder, paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting any excess drip off, and dredge in the panko mixture, pressing firmly to ensure an even coating.
Preheat the air fryer to 375°F and lightly coat the air fryer basket with avocado oil spray.
Place the chicken in the basket, spray the top of the chicken with avocado oil, and air-fry for 12-15 minutes, flipping halfway through, until the exterior is golden brown and the internal temperature reaches 165°F.
While the chicken cooks, whisk together the Greek yogurt and Dijon mustard in a small bowl.
Toast the whole wheat bun until lightly browned.
Assemble the sandwich by spreading the yogurt-mustard sauce on the bun, then layering the lettuce, tomato, and crispy chicken.