Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness.
In a small bowl, whisk together the Greek yogurt and lemon juice to create a clean buttermilk alternative.
In a separate shallow dish, combine the arrowroot starch, almond flour, garlic powder, onion powder, sea salt, and black pepper.
Dip the chicken breast into the yogurt mixture until fully coated, then dredge it through the flour mixture, pressing firmly to adhere the crust.
Heat the avocado oil in a medium skillet over medium-high heat until shimmering.
Carefully place the chicken in the skillet and cook for 4-5 minutes per side until the exterior is golden brown and the internal temperature reaches 165°F.
While the chicken rests, lightly toast the sprouted grain bun in the same skillet or a toaster.
Assemble the sandwich by placing the lettuce and tomato on the bottom bun, followed by the crispy chicken, and finish with the top bun.