Place the chicken breast between two sheets of parchment paper and pound gently to an even half-inch thickness.
In a shallow bowl, submerge the chicken in the buttermilk and let it 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 thoroughly in the flour mixture, pressing it into the meat to ensure a thick coating.
Heat the avocado oil in a cast-iron or non-stick skillet over medium-high heat.
Once the oil is shimmering, add the chicken and cook for 5-6 minutes per side until the exterior is golden brown and the internal temperature reaches 165°F.
While the chicken rests, stir together the Greek yogurt and Dijon mustard in a small bowl to create a healthy spread.
Lightly toast the whole wheat bun, spread the yogurt-mustard sauce on both sides, and assemble with the crispy chicken, lettuce, and tomato.