Place the chicken breast between two sheets of parchment paper and pound with a meat mallet until it is an even half-inch thickness.
Place the chicken in a shallow bowl and pour the buttermilk over it, ensuring it is fully submerged. Let it marinate for at least 15 minutes.
In a separate shallow dish, whisk together the whole wheat flour, sea salt, black pepper, paprika, and garlic powder.
Remove the chicken from the buttermilk, letting any excess drip off, then dredge it thoroughly in the flour mixture until well-coated.
Heat the avocado oil in a non-stick skillet over medium-high heat.
Place the chicken in the skillet and cook for 4-5 minutes per side, or until the coating is golden brown and the internal temperature reaches 165°F.
While the chicken cooks, whisk together the Greek yogurt and Dijon mustard in a small bowl to create a healthy sandwich spread.
Lightly toast the whole wheat bun in the skillet or a toaster.
Spread the yogurt-mustard sauce on both sides of the bun, then layer the romaine lettuce, tomato slices, crispy chicken, and pickle chips to assemble the sandwich.