Place the chicken breast between two sheets of parchment paper and pound to an even 1/2-inch thickness.
In a shallow bowl, submerge the chicken in the buttermilk and let it marinate for at least 15 minutes.
In a separate shallow dish, whisk together the whole wheat flour, smoked paprika, garlic powder, onion powder, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting excess drip off, then dredge thoroughly in the flour mixture, pressing down to ensure a thick coating.
Lightly spray or brush 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, stir together the Greek yogurt and Dijon mustard in a small bowl to create the sandwich spread.
Lightly toast the whole wheat bun, then assemble the sandwich by layering the yogurt spread, romaine lettuce, tomato slices, and the crispy chicken.