Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness for uniform cooking.
In a shallow bowl, whisk together the buttermilk, smoked paprika, garlic powder, sea salt, and black pepper.
Submerge the chicken in the buttermilk mixture and let it marinate for at least 15 minutes in the refrigerator.
Remove the chicken from the marinade and dredge lightly in the whole wheat flour, shaking off any excess coating.
Heat the avocado oil in a non-stick skillet over medium-high heat until it begins to shimmer.
Cook the chicken for 5 to 6 minutes per side until the exterior is golden brown and the internal temperature reaches 165°F.
In a small ramekin, stir together the Greek yogurt and Dijon mustard to create a creamy, zesty sandwich spread.
Lightly toast the whole wheat bun in the skillet or a toaster until the edges are fragrant and slightly browned.
Assemble the sandwich by spreading the yogurt sauce on the bun, then layering with lettuce, tomato, the crispy chicken, and pickles.