Slice the chicken breast horizontally to create a thinner cutlet for even cooking.
In a shallow bowl, soak the chicken in buttermilk for at least 15 minutes to tenderize.
In another bowl, whisk together the whole wheat flour, garlic powder, paprika, sea salt, and black pepper.
Dredge the buttermilk-soaked chicken in the flour mixture, pressing firmly to ensure a thick, even coating.
Heat avocado oil in a cast-iron skillet over medium-high heat until shimmering.
Carefully place the chicken in the skillet and cook for 4-5 minutes per side until the crust is golden-brown and the internal temperature reaches 165°F.
While the chicken cooks, whisk the Greek yogurt and Dijon mustard together in a small ramekin to create a clean-eating spread.
Toast the sprouted grain bun in the same skillet or a toaster until lightly browned and fragrant.
Assemble the sandwich by spreading the yogurt-mustard sauce on the bun, then layering the lettuce, crispy chicken, and pickles.