Place the chicken breast between two sheets of parchment paper and pound to an even thickness of about half an inch.
In a shallow bowl, combine the chicken and buttermilk, ensuring the meat is fully coated; let it marinate for at least 15 minutes.
In a separate small bowl, whisk together the whole wheat flour, sea salt, black pepper, garlic powder, and smoked paprika.
Remove the chicken from the buttermilk, allowing excess to drip off, then dredge thoroughly in the flour mixture until evenly coated.
Heat the avocado oil in a non-stick skillet over medium heat.
Place the chicken in the skillet and cook for 5-6 minutes per side until the exterior is golden brown and the internal temperature reaches 165°F.
While the chicken cooks, lightly toast the sprouted grain bun in a toaster or on a separate dry pan.
Spread the Dijon mustard on the bottom half of the toasted bun.
Layer the romaine lettuce and tomato slices on the bun, top with the crispy chicken, and close the sandwich.