Place the chicken breast in a shallow bowl and pour the buttermilk over it, letting it marinate for at least 20 minutes to tenderize.
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 seasoned flour mixture until coated.
Lightly coat the air fryer basket with half of the avocado oil and place the chicken inside, spraying the top with the remaining oil.
Air fry at 375°F for 12-15 minutes, flipping halfway through, until the exterior is golden and the internal temperature reaches 165°F.
While the chicken cooks, mix the Greek yogurt and Dijon mustard in a small ramekin to create a creamy, high-protein sandwich spread.
Lightly toast the whole wheat bun and assemble the sandwich with the yogurt sauce, lettuce, pickles, and the crispy chicken breast.