Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness to ensure quick and even cooking.
Season both sides of the chicken evenly with the sea salt, black pepper, garlic powder, and smoked paprika.
Heat the olive oil in a non-stick 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, lightly toast the whole grain bun in a separate pan or toaster until warm and slightly crisp.
In a small bowl, whisk together the Greek yogurt and Dijon mustard to create a clean, high-protein spread.
Slice the avocado into thin wedges and prepare the tomato slices.
Spread the yogurt-mustard mixture on the bottom bun, then layer with the arugula, tomato slices, seared chicken, and avocado.
Place the top bun on the sandwich and serve immediately while the chicken is hot and juicy.