Place the chicken breast in a shallow bowl and cover with buttermilk, marinating for at least 15 minutes.
In a separate small bowl, whisk together the whole wheat flour, panko breadcrumbs, smoked paprika, garlic powder, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting any excess drip off, then dredge thoroughly in the flour mixture until evenly coated.
Preheat your air fryer to 375°F and lightly spray or brush the chicken with avocado oil.
Air fry the chicken for 12-15 minutes, flipping halfway through, until the internal temperature reaches 165°F and the exterior is golden brown.
While the chicken cooks, stir together the Greek yogurt and Dijon mustard in a small ramekin to create a healthy spread.
Toast the whole grain bun lightly in a pan or toaster.
Assemble the sandwich by spreading the yogurt-mustard sauce on the bun, then layering the lettuce, tomato, and crispy chicken.