Place the chicken breast in a shallow bowl and pour the buttermilk over it, ensuring it is fully coated; let it marinate for at least 15 minutes.
In a separate small bowl, whisk together the whole wheat flour, smoked paprika, garlic powder, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting excess drip off, then dredge it through the flour mixture until evenly coated on both sides.
Heat the avocado oil in a non-stick skillet over medium-high heat.
Add the chicken to the skillet and cook for 5-6 minutes per side, or until the exterior is golden brown and the internal temperature reaches 165°F.
While the chicken cooks, lightly toast the whole wheat bun in a toaster or under a broiler.
In a tiny bowl, stir together the Greek yogurt and Dijon mustard to create a clean-eating spread.
Assemble the sandwich by spreading the yogurt-mustard sauce on the bottom bun, followed by the lettuce, the crispy chicken, and the dill pickles before topping with the other half of the bun.