Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness.
Submerge the chicken in buttermilk in a shallow bowl and marinate for at least 15 minutes.
In a separate shallow dish, 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 in the flour mixture, pressing firmly to ensure an even coating.
Heat the avocado oil in a cast-iron skillet over medium-high heat until shimmering.
Place the chicken in the skillet and sear for 5-6 minutes per side until the crust is crunchy and the internal temperature reaches 165°F.
While the chicken rests, whisk the Greek yogurt and Dijon mustard together in a small bowl to create a clean-eating spread.
Toast the whole wheat bun, spread the yogurt sauce on both sides, and assemble with the romaine lettuce, seared chicken, and dill pickles.