Place the chicken breast between two sheets of parchment paper and pound to an even thickness of about half an inch.
In a shallow bowl, soak the chicken in the buttermilk for at least 10 minutes to tenderize.
In a separate shallow dish, combine the panko breadcrumbs, almond flour, garlic powder, onion powder, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting excess drip off, then dredge thoroughly in the breadcrumb mixture, pressing to adhere.
Lightly coat the breaded chicken with avocado oil spray and place in an air fryer at 375°F for 12-15 minutes, flipping halfway through, until golden and crispy.
While the chicken cooks, whisk together the Greek yogurt and Dijon mustard in a small bowl to create a clean-eating spread.
Toast the whole grain bun until lightly browned.
Assemble the sandwich by spreading the yogurt mixture on the bun, then layering the crispy chicken, lettuce, and tomato slices.