Place the chicken breast in a shallow bowl and pour the buttermilk over it, letting it marinate for at least 30 minutes in the refrigerator.
In a separate shallow dish, whisk together the almond flour, cornstarch, garlic powder, onion powder, smoked paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting any excess drip off, then dredge it through the flour mixture, pressing firmly to ensure an even coating.
Lightly spray the chicken on both sides with avocado oil and place it in the air fryer basket at 375°F for 12 to 15 minutes, flipping halfway through until golden and crispy.
While the chicken cooks, stir together the Greek yogurt and Dijon mustard in a small bowl to create a clean, high-protein sandwich spread.
Toast the sprouted grain bun until lightly browned, then assemble the sandwich with the yogurt sauce, lettuce, tomato, pickles, and the crispy chicken.