Slice the chicken breast into a flat cutlet and marinate in buttermilk for at least 30 minutes in the refrigerator.
Whisk together the whole wheat flour, smoked paprika, garlic powder, onion powder, sea salt, and black pepper in a shallow bowl.
Remove the chicken from the buttermilk and dredge it in the flour mixture, pressing firmly to ensure an even, crunchy coating.
Lightly coat the breaded chicken with avocado oil spray and place it in the air fryer basket.
Air fry at 400°F for 12-15 minutes, flipping halfway through, until the exterior is golden brown and the internal temperature reaches 165°F.
In a small bowl, mix the plain Greek yogurt and Dijon mustard together to create a creamy, high-protein sandwich spread.
Toast the whole grain bun until the edges are lightly browned and fragrant.
Assemble the sandwich by spreading the yogurt sauce on both sides of the bun, then layering the chicken, romaine lettuce, tomato slices, and pickles.