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 for at least 20 minutes.
In a separate bowl, whisk together the almond flour, arrowroot powder, garlic powder, onion powder, smoked paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting excess drip off, then dredge thoroughly in the flour mixture, pressing to adhere.
Lightly coat the chicken with avocado oil and air-fry at 375°F for 12-15 minutes, flipping halfway through, until the exterior is golden and crisp.
While the chicken cooks, mix the Greek yogurt and Dijon mustard in a small bowl to create the sandwich sauce.
Toast the sprouted grain bun until lightly browned.
Assemble the sandwich by spreading the yogurt sauce on the bun, then layering the lettuce, tomato, and crispy chicken.