Place the chicken breast between two sheets of parchment paper and pound with a meat mallet until it is an even half-inch thickness.
Place the chicken in a shallow bowl and pour the buttermilk over it, ensuring it is fully coated; let it marinate for at least 20 minutes in the refrigerator.
In a small plate, whisk together the whole wheat flour, garlic powder, onion powder, sea salt, and black pepper.
Remove the chicken from the buttermilk, allowing excess to drip off, then dredge it through the flour mixture until lightly and evenly coated.
Heat the avocado oil in a non-stick skillet over medium-high heat.
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, lightly toast the whole wheat bun in the same skillet or a toaster.
Assemble the sandwich by placing the lettuce and tomato on the bottom bun, followed by the crispy chicken, and finishing with the dill pickles and the top bun.