Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness.
In a shallow bowl, submerge the chicken in buttermilk and let marinate for at least 15 minutes in the refrigerator.
In a separate small bowl, whisk together the whole wheat flour, garlic powder, onion powder, sea salt, black pepper, and smoked paprika.
Remove the chicken from the buttermilk, allowing excess to drip off, then dredge thoroughly in the flour mixture until fully coated.
Heat the avocado oil in a non-stick skillet over medium-high heat.
Place the chicken in the skillet and cook for 4 to 5 minutes per side until the coating is golden brown and the internal temperature reaches 165 degrees Fahrenheit.
While the chicken cooks, lightly toast the whole grain bun in a toaster or under a broiler.
Assemble the sandwich by placing the cooked chicken on the bottom bun, followed by the lettuce, tomato slices, and red onion, then top with the remaining bun half.