Place the chicken breast between two sheets of parchment paper and pound with a meat mallet until it is an even half-inch thickness.
Submerge the chicken in the low-fat buttermilk in a shallow bowl and let it marinate for at least 20 minutes in the refrigerator.
In a separate shallow dish, whisk together the whole wheat flour, garlic powder, onion powder, paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, allowing excess to drip off, then dredge it thoroughly in the spiced flour mixture, pressing firmly to ensure a thick coating.
Preheat your air fryer to 375°F and lightly mist the air fryer basket with olive oil.
Place the coated chicken in the basket and spray the top of the chicken with the remaining olive oil to help it crisp up.
Air fry for 12 to 15 minutes, flipping the chicken halfway through, until the internal temperature reaches 165°F and the exterior is golden brown.
While the chicken cooks, whisk together the Greek yogurt and Dijon mustard in a small bowl to create a clean-eating spread.
Lightly toast the whole wheat bun until warm and golden.
Assemble the sandwich by spreading the yogurt sauce on the bun, then layering with the romaine lettuce, tomato slices, and the crispy chicken breast.