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, letting it marinate for at least 15 minutes.
In a separate small bowl, whisk together the whole wheat flour, garlic powder, onion powder, sea salt, and black pepper.
Remove the chicken from the buttermilk, allowing excess to drip off, then dredge thoroughly in the flour mixture, pressing firmly to adhere.
Heat the avocado oil in a non-stick skillet over medium-high heat or preheat an air fryer to 375°F.
Cook the chicken for 5-6 minutes per side in the skillet, or 12-15 minutes in the air fryer, until the crust is golden and the internal temperature reaches 165°F.
Lightly toast the whole grain bun until the edges are slightly browned and fragrant.
Layer the bottom bun with romaine lettuce and tomato, top with the crispy chicken and pickles, and finish with the top bun.