Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness.
Place the chicken in a shallow bowl, cover with buttermilk, and marinate for at least 20 minutes in the refrigerator.
In a separate shallow dish, whisk together the arrowroot powder, garlic powder, onion powder, paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, allowing the excess to drip off, and dredge it thoroughly in the seasoned arrowroot mixture.
Heat the avocado oil in a cast-iron skillet over medium-high heat until the oil is shimmering.
Carefully place the chicken in the skillet and cook for 4-5 minutes per side until the exterior is golden-brown and the internal temperature reaches 165°F.
While the chicken rests for 2 minutes, lightly toast the sprouted grain bun in the skillet or a toaster.
Assemble the sandwich by placing the crispy chicken on the bottom bun, followed by the lettuce, tomato slices, and pickles.