Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness.
In a shallow bowl, soak the chicken in buttermilk for at least 15 minutes to tenderize.
In a separate shallow dish, whisk together the panko, flour, garlic powder, onion powder, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting the excess drip off, then dredge thoroughly in the panko mixture, pressing firmly to adhere.
Heat avocado oil in a non-stick skillet over medium heat. Pan-fry the chicken for 4-5 minutes per side until golden brown and cooked through.
While the chicken cooks, whisk the Greek yogurt and Dijon mustard together in a small bowl to create a clean-eating spread.
Toast the whole wheat bun lightly in the skillet or a toaster.
Assemble the sandwich by spreading the yogurt-mustard sauce on the bun, then layering the lettuce, crispy chicken, and pickles.