Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness.
Submerge the chicken in a bowl with the buttermilk and let it marinate for at least 15 minutes.
In a shallow dish, whisk together the flour, cornstarch, garlic powder, onion powder, smoked paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, allowing excess to drip off, then dredge thoroughly in the flour mixture until fully coated.
Lightly coat the air fryer basket with avocado oil and place the chicken inside, spraying the top of the chicken with the remaining oil.
Air fry at 400 degrees Fahrenheit for 12 to 15 minutes, flipping halfway through, until the exterior is golden and the internal temperature reaches 165 degrees Fahrenheit.
While the chicken cooks, whisk together the Greek yogurt and Dijon mustard in a small bowl to create the sauce.
Toast the bun until lightly browned, then spread the sauce on both sides.
Assemble the sandwich by placing the crispy chicken on the bottom bun, followed by the lettuce and tomato slices, and top with the remaining bun.