Place the chicken breast between two sheets of parchment paper and pound to an even thickness for uniform cooking.
Submerge the chicken in buttermilk in a shallow bowl and marinate for at least 30 minutes to ensure a tender texture.
In a separate bowl, whisk together the almond flour, garlic powder, onion powder, paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting the excess liquid drip off, then dredge thoroughly in the seasoned flour mixture.
Lightly coat the air fryer basket with avocado oil and place the breaded chicken inside.
Air fry at 375°F for 12-15 minutes, flipping the chicken halfway through, until the exterior is golden and crispy.
While the chicken cooks, whisk the Greek yogurt and Dijon mustard together in a small bowl to create a clean, tangy spread.
Toast the whole grain bun and assemble the sandwich by layering the mustard sauce, crispy chicken, and pickles.