Preheat your air fryer to 400°F.
Pound the chicken breast between two pieces of parchment paper until it reaches an even thickness.
Brush both sides of the chicken breast with the Dijon mustard to act as a binder.
In a shallow dish, whisk together the almond flour, smoked paprika, garlic powder, sea salt, and black pepper.
Press the mustard-coated chicken firmly into the almond flour mixture until it is evenly and thoroughly coated.
Lightly spray the chicken on both sides with avocado oil and place it in the air fryer basket.
Air fry for 12-15 minutes, flipping halfway through, until the coating is golden and the internal temperature reaches 165°F.
Lightly toast the whole grain bun until the edges are golden brown.
Spread the avocado oil mayo on the bottom bun, then layer with romaine lettuce, tomato slices, and the crispy chicken breast before closing the sandwich.