Place the chicken breast between two pieces of parchment paper and pound to an even 1/2-inch thickness for uniform cooking.
In a shallow bowl, whisk together the buttermilk, garlic powder, smoked paprika, sea salt, and black pepper to create the marinade.
Submerge the chicken in the buttermilk mixture and let it marinate for at least 15 minutes to tenderize the meat.
Place the whole wheat flour on a small plate and dredge the marinated chicken, pressing lightly to ensure the flour adheres to the surface.
Heat the avocado oil in a non-stick skillet over medium-high heat until the oil is shimmering.
Carefully place the chicken in the skillet and cook for 4-5 minutes per side until the exterior is golden brown and the internal temperature reaches 165°F.
While the chicken cooks, toast the whole wheat bun in a toaster or a separate dry pan until the edges are lightly browned and crisp.
In a small ramekin, stir together the plain Greek yogurt and Dijon mustard until smooth to create a high-protein sandwich spread.
Spread the yogurt-mustard sauce evenly on both the top and bottom halves of the toasted bun.
Assemble the sandwich by layering the romaine lettuce and tomato slices on the bottom bun, followed by the crispy chicken and pickle chips.