Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness.
In a shallow bowl, whisk together the buttermilk, garlic powder, onion powder, smoked paprika, salt, and pepper.
Place the panko breadcrumbs in a separate shallow dish.
Dip the chicken into the buttermilk mixture, coating both sides, then dredge thoroughly in the panko breadcrumbs, pressing firmly so they adhere.
Preheat your air fryer to 375 degrees Fahrenheit and lightly spray the basket with avocado oil.
Place the chicken in the air fryer and spray the top of the breading with avocado oil; cook for 12-15 minutes, flipping halfway through, until the exterior is golden and the internal temperature reaches 165 degrees Fahrenheit.
While the chicken cooks, whisk the Greek yogurt and Dijon mustard together in a small bowl to create a clean sandwich spread.
Lightly toast the sprouted grain bun in a pan or toaster.
Assemble the sandwich by spreading the yogurt sauce on the bun, then layering the lettuce, tomato, crispy chicken, and pickles.