Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness.
Place the chicken in a shallow bowl and pour the buttermilk over it, ensuring it is fully submerged to 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 the excess drip off, then dredge it through the flour mixture until evenly coated.
Heat the avocado oil in a non-stick skillet over medium heat and cook the chicken for 5-6 minutes per side until the crust is golden brown and the internal temperature reaches 165 degrees.
While the chicken cooks, lightly toast the whole grain bun in a toaster or on a dry pan.
In a small bowl, stir together the Greek yogurt and Dijon mustard to create a healthy sandwich spread.
Spread the yogurt mixture onto the toasted bun, then layer the lettuce, tomato, pickles, and the crispy chicken breast to assemble the sandwich.