Place the chicken breast between two pieces of parchment paper and pound to an even thickness of about half an inch.
In a shallow bowl, soak the chicken in the buttermilk for at least 15 minutes to tenderize the meat.
In a separate small bowl, whisk together the flour, smoked paprika, garlic powder, onion powder, sea salt, and black pepper.
Remove the chicken from the buttermilk and dredge it in the flour mixture, pressing firmly to ensure a thick, even coating on all sides.
Lightly spray both sides of the coated chicken with avocado oil and place in the air fryer basket.
Air-fry at 375°F for 12-15 minutes, flipping halfway through, until the exterior is golden brown and the internal temperature reaches 165°F.
While the chicken cooks, toast the whole wheat bun in a dry pan or toaster until warm.
In a small ramekin, stir together the Greek yogurt and Dijon mustard to create a clean-label sandwich spread.
Assemble the sandwich by spreading the yogurt sauce on the bottom bun, followed by the lettuce, tomato slices, and the crispy chicken.