Place the chicken breast between parchment paper and pound to an even half-inch thickness.
Submerge the chicken in buttermilk in a shallow bowl and let it marinate for 10 minutes.
In a separate bowl, whisk together arrowroot powder, smoked paprika, garlic powder, onion powder, salt, and pepper.
Remove chicken from buttermilk and dredge thoroughly in the arrowroot mixture until fully coated.
Heat avocado oil in a skillet over medium-high heat until shimmering.
Carefully place chicken in the pan and cook for 4 to 5 minutes per side until the crust is golden and crispy.
While chicken cooks, lightly toast the sprouted grain bun in a separate pan or toaster.
Mix the Greek yogurt and Dijon mustard in a small ramekin to create a clean-label sandwich spread.
Spread the yogurt mixture on the bottom bun, then layer with the lettuce leaf and crispy chicken.
Top with pickle slices and the remaining bun half before serving immediately.