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 thoroughly with the sea salt, black pepper, garlic powder, and paprika.
Heat the avocado oil in a stainless steel or cast-iron skillet over medium-high heat until the oil shimmers.
Carefully place the chicken in the pan and sear for 5 to 6 minutes per side until a golden crust forms and the center is cooked through.
In a small ramekin, stir together the Greek yogurt and Dijon mustard until the sauce is smooth and combined.
Lightly toast the whole wheat bun in the same skillet for 1 minute until the edges are warm and slightly crunchy.
Spread the yogurt-mustard sauce on the bottom bun, then layer with the lettuce, tomato, seared chicken, and pickles to serve.