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 whole wheat flour, garlic powder, onion powder, paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting excess drip off, then dredge in the flour mixture, pressing firmly to ensure a light coating.
Heat avocado oil in a non-stick skillet over medium heat.
Add the chicken to the skillet and cook for 5-6 minutes per side until the exterior is golden and the internal temperature reaches 165 degrees.
While the chicken cooks, whisk together the Greek yogurt and mustard in a small bowl to create a clean spread.
Lightly toast the whole wheat bun in the skillet or a toaster until warm.
Assemble the sandwich by spreading the yogurt mustard on the bun, then layering the lettuce, crispy chicken, and pickles.