Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness.
Submerge the chicken in buttermilk in a shallow bowl and marinate in the refrigerator for at least 30 minutes.
In a separate small bowl, whisk together the whole wheat flour, garlic powder, paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, allowing the excess to drip off, then dredge it through the flour mixture until fully coated.
Lightly spray the chicken on both sides with olive oil and place in the air fryer basket.
Air fry at 375°F for 12 to 15 minutes, flipping halfway through, until the exterior is golden and the internal temperature reaches 165°F.
While the chicken cooks, stir together the Greek yogurt and Dijon mustard to create a healthy sandwich spread.
Toast the whole wheat bun lightly, then assemble the sandwich by layering the sauce, lettuce, tomato, and the crispy chicken fillet.