Place the chicken breast between two sheets of parchment paper and pound to an even 1/2-inch thickness.
In a shallow bowl, combine the chicken and buttermilk; let marinate for at least 15 minutes.
In a separate bowl, whisk together the whole wheat flour, garlic powder, onion powder, paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting excess drip off, then dredge thoroughly in the flour mixture.
Lightly coat the chicken with the avocado oil using a brush or spray and air fry at 400°F for 12-15 minutes, flipping halfway, until golden and crispy.
While the chicken cooks, mix the Greek yogurt and Dijon mustard in a small bowl to create a zesty spread.
Toast the sprouted grain bun in a dry pan or toaster until lightly browned.
Assemble the sandwich by spreading the yogurt sauce on the bun, then layering the lettuce, tomato, crispy chicken, and pickles.