Place the chicken breast in a shallow bowl and pour the buttermilk over it, ensuring it is fully submerged; marinate for at least 20 minutes.
In a separate small bowl, whisk together the whole wheat flour, garlic powder, onion powder, paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting the excess drip off, then dredge it thoroughly in the flour mixture until evenly coated.
Lightly coat the air fryer basket with avocado oil and place the chicken inside, spraying the top of the chicken with the remaining oil.
Air fry at 375 degrees Fahrenheit for 12 to 15 minutes, flipping halfway through, until the internal temperature reaches 165 degrees and the crust is golden brown.
While the chicken cooks, mix the Greek yogurt and Dijon mustard in a small ramekin to create a healthy spread.
Toast the whole wheat bun lightly, then assemble the sandwich by spreading the yogurt sauce on the bottom bun and layering with the crispy chicken, lettuce, tomato, and pickles.