Place the chicken breast between two sheets of parchment paper and pound with a meat mallet until it reaches an even half-inch thickness.
In a shallow bowl, whisk together the buttermilk, half of the garlic powder, half of the onion powder, and a pinch of salt and pepper.
Submerge the chicken in the buttermilk mixture and let it marinate for at least 15 minutes in the refrigerator.
In a separate shallow dish, combine the whole wheat flour with the remaining garlic powder, onion powder, sea salt, and black pepper.
Remove the chicken from the buttermilk, allowing excess to drip off, then dredge thoroughly in the flour mixture, pressing firmly to ensure a thick coating.
Heat the avocado oil in a non-stick skillet over medium-high heat until shimmering.
Add the chicken to the pan and cook for 4 to 5 minutes per side, or until the coating is deeply golden and the internal temperature reaches 165°F.
While the chicken rests for 2 minutes, 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 pickles, then top with the remaining bun half.