Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness.
Place chicken in a shallow bowl and cover with buttermilk, marinating for at least 15 minutes in the refrigerator.
In a separate shallow dish, whisk together the almond flour, arrowroot powder, garlic powder, smoked paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting the excess drip off, then dredge thoroughly in the flour mixture until fully coated.
Lightly spray the air fryer basket with avocado oil and place the chicken inside, spraying the top of the chicken with the remaining oil.
Air fry at 375°F for 12-15 minutes, flipping halfway through, until the coating is golden brown and the internal temperature reaches 165°F.
Toast the sprouted grain bun lightly if desired.
Assemble the sandwich by spreading Dijon mustard on the bottom bun, followed by the lettuce, tomato, crispy chicken, and the top bun.