Place the chicken breast between two sheets of parchment paper and pound with a meat mallet to an even half-inch thickness.
Place the chicken in a shallow bowl and pour the buttermilk over it, ensuring it is fully submerged; marinate in the refrigerator for 30 minutes.
In a separate shallow dish, whisk together the whole wheat flour, garlic powder, onion powder, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting excess drip off, then dredge it in the flour mixture, pressing firmly to create a solid coating.
Heat the avocado oil in a non-stick skillet over medium-high heat until shimmering.
Place the chicken in the skillet and cook for 5 to 6 minutes per side until the exterior is golden brown and the internal temperature reaches 165°F.
While the chicken rests for 2 minutes, lightly toast the whole wheat bun in the same skillet or a toaster.
Spread the Dijon mustard on the bottom bun, top with the crispy chicken and pickle slices, and close the sandwich.