Place the chicken breast between two sheets of parchment paper and pound with a meat mallet until it reaches an even 1/2-inch thickness.
In a shallow bowl, submerge the chicken in buttermilk and let it marinate in the refrigerator for at least 30 minutes to tenderize.
In a separate shallow dish, whisk together the whole wheat flour, garlic powder, onion powder, smoked paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, allowing excess to drip off, then dredge it thoroughly in the flour mixture, pressing down firmly to ensure a thick coating.
Heat the avocado oil in a cast-iron or non-stick skillet over medium-high heat until shimmering.
Carefully place the chicken in the skillet and sear for 4-5 minutes per side until the crust is golden brown and the internal temperature reaches 165°F.
While the chicken rests for 2 minutes, lightly toast the whole-grain bun in the same skillet until warmed through.
Assemble the sandwich by placing the crisp lettuce and tomato on the bottom bun, followed by the chicken and pickles, then top with the remaining bun half.