Place the chicken breast between two sheets of parchment paper and pound to an even thickness of about half an inch for uniform cooking.
In a small bowl, combine the smoked paprika, garlic powder, sea salt, and black pepper.
Rub the spice mixture evenly over both sides of the chicken breast, pressing it into the meat.
Heat the avocado oil in a cast-iron or stainless steel skillet over medium-high heat until shimmering.
Add the chicken to the pan and sear for 5-6 minutes per side until the exterior is golden-brown and the internal temperature reaches 165°F.
While the chicken cooks, whisk together the Greek yogurt and Dijon mustard in a small ramekin to create a clean-eating spread.
Lightly toast the sprouted grain bun in the same pan or a toaster until warm and slightly crisp.
Spread the yogurt-mustard mixture on the bottom bun, then layer with the lettuce, tomato, and red onion.
Place the hot seared chicken on top of the vegetables, add the top bun, and serve immediately.