Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness using a meat mallet.
Whisk the buttermilk with sea salt, black pepper, and paprika in a shallow bowl; submerge the chicken and marinate for 20 minutes.
In another shallow dish, whisk together the whole wheat flour, garlic powder, and onion powder.
Remove the chicken from the buttermilk, allowing the excess to drip off, then dredge thoroughly in the flour mixture until fully coated.
Heat the olive oil in a non-stick skillet over medium-high heat.
Cook the chicken for 4-5 minutes per side until the crust is golden-brown and the internal temperature reaches 165°F.
While the chicken cooks, mix the Greek yogurt and Dijon mustard in a small bowl to create a creamy spread.
Lightly toast the whole wheat bun, spread the yogurt sauce on both sides, and assemble the sandwich with lettuce, tomato, and the crispy chicken.