Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness.
In a shallow dish, whisk together the almond flour, garlic powder, onion powder, sea salt, and black pepper.
Press the chicken breast firmly into the flour mixture on both sides until thoroughly and evenly coated.
Lightly coat the chicken with avocado oil and place in an air fryer basket at 400F for 12 minutes, flipping halfway through, until the exterior is golden and crisp.
While the chicken cooks, whisk together the Dijon mustard, honey, and Greek yogurt in a small bowl to create the sauce.
Toast the whole-wheat bun in a dry skillet over medium heat until the edges are lightly browned.
Spread half of the honey-mustard sauce on the bottom bun, then layer with the romaine lettuce and tomato slices.
Place the crispy chicken on top, drizzle with the remaining sauce, and close the sandwich with the top bun.