Place the chicken breast in a bowl and pour the buttermilk over it, ensuring it is fully submerged; marinate for at least 30 minutes.
In a shallow dish, whisk together the whole wheat flour, garlic powder, onion powder, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting the excess drip off, and dredge it in the flour mixture, pressing firmly to create an even coating.
Lightly brush or spray the coated chicken with avocado oil and place it in the air fryer basket.
Air-fry at 375°F for 12-15 minutes, flipping halfway through, until the exterior is golden brown and the internal temperature reaches 165°F.
While the chicken cooks, lightly toast the sprouted grain bun in a pan or toaster.
In a small ramekin, stir together the Greek yogurt and Dijon mustard to create a creamy spread.
Spread the yogurt-mustard sauce on the toasted bun, then layer with romaine lettuce, tomato slices, and the crispy chicken breast.