Butterfly the chicken breast by slicing it horizontally to ensure even cooking and a faster air-fry time.
In a shallow bowl, whisk together the Greek yogurt and water to create a clean-eating buttermilk substitute.
In a separate shallow dish, combine the almond flour, arrowroot powder, garlic powder, onion powder, sea salt, and black pepper.
Dip the chicken breast into the yogurt mixture until fully coated, then dredge it through the flour mixture, pressing firmly to adhere.
Lightly brush or spray the coated chicken with avocado oil and place in the air fryer basket.
Air-fry at 375°F for 12-15 minutes, flipping halfway through, until the exterior is golden and the internal temperature reaches 165°F.
Toast the sprouted grain bun and assemble the sandwich with the crispy chicken, lettuce, tomato, and a spread of Dijon mustard.