Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness.
Submerge the chicken in buttermilk in a shallow bowl and marinate in the refrigerator for at least 30 minutes.
In a separate shallow dish, whisk together the whole wheat flour, garlic powder, onion powder, paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, allowing excess to drip off, then dredge thoroughly in the flour mixture until fully 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 exterior is golden and the internal temperature reaches 165 degrees.
Whisk the Greek yogurt and Dijon mustard together in a small bowl to create a creamy spread.
Lightly toast the bun, spread the yogurt sauce on both sides, and assemble with the chicken, lettuce, and tomato slices.