Place the chicken breast in a shallow bowl and cover with buttermilk, letting it marinate for at least 20 minutes.
In a separate small bowl, whisk together the whole wheat flour, smoked paprika, garlic powder, onion powder, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting the excess drip off, then dredge it thoroughly in the flour mixture until fully coated.
Lightly coat the air fryer basket with half of the avocado oil and place the chicken inside, spraying the top of the chicken with the remaining oil.
Air fry at 375 degrees Fahrenheit for 12 to 15 minutes, flipping halfway through, until the coating is golden and the internal temperature reaches 165 degrees.
While the chicken cooks, stir together the Greek yogurt and Dijon mustard in a small ramekin to create a clean-eating spread.
Toast the whole wheat bun and assemble the sandwich with the yogurt sauce, cooked chicken, romaine lettuce, and tomato slices.