Place the chicken breast between two sheets of parchment paper and pound with a meat mallet until it is an even half-inch thickness.
In a shallow bowl, pour the buttermilk over the chicken and turn to coat completely.
In a separate shallow dish, whisk together the almond flour, arrowroot starch, smoked paprika, garlic powder, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting any excess drip off, then dredge thoroughly in the flour mixture, pressing to ensure the coating adheres.
Heat the avocado oil in a non-stick skillet over medium-high heat.
Place the chicken in the hot skillet and cook for 4-5 minutes per side, or until the exterior is golden-brown and the internal temperature reaches 165°F.
While the chicken cooks, lightly toast the whole grain bun in a toaster or on a separate dry pan.
Spread the Dijon mustard on the bottom bun, then layer the romaine lettuce and dill pickles.
Place the crispy chicken on top of the pickles and close the sandwich with the top bun.