Place the chicken breast in a shallow bowl and cover with buttermilk to marinate for 15 minutes.
In a separate small bowl, combine the almond flour, sea salt, black pepper, garlic powder, and smoked paprika.
Remove the chicken from the buttermilk and dredge it through the almond flour mixture until fully coated.
Preheat an air fryer to 375°F and lightly spray the basket with avocado oil.
Place the chicken in the air fryer and cook for 12-15 minutes, flipping halfway through, until the exterior is golden and the internal temperature reaches 165°F.
While the chicken cooks, whisk together the Greek yogurt and Dijon mustard in a small ramekin to create the sauce.
Toast the whole grain bun until lightly browned.
Spread the yogurt-mustard sauce on the bun, then layer with the lettuce, tomato, and the crispy chicken breast.