Place the chicken breast in a shallow bowl and pour the buttermilk over it, ensuring it is fully submerged. Let it marinate for at least 15 minutes.
In a separate small bowl, combine the panko breadcrumbs, garlic powder, paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting any excess drip off, then dredge it thoroughly in the panko mixture until evenly coated.
Preheat your air fryer to 375 degrees F. Lightly spray or brush the chicken with avocado oil.
Place the chicken in the air fryer basket and cook for 12-15 minutes, flipping halfway through, until the internal temperature reaches 165 degrees F and the crust is golden brown.
While the chicken cooks, whisk together the Greek yogurt and Dijon mustard in a small ramekin to create a clean-eating spread.
Lightly toast the whole wheat bun in a toaster or dry pan.
Assemble the sandwich by spreading the yogurt-mustard sauce on the bottom bun, followed by the crispy chicken, lettuce, and tomato slices.