Season the chicken breast evenly on both sides with the sea salt, black pepper, garlic powder, and dried oregano.
Heat the olive oil in a medium non-stick skillet over medium-high heat until the oil is shimmering.
Place the chicken breast in the skillet and sear for 5 to 7 minutes per side, or until the internal temperature reaches 165 degrees Fahrenheit and the exterior is golden brown.
While the chicken is resting, lightly toast the brioche bun halves in a toaster or a clean pan until the edges are slightly crisp.
In a small bowl, stir the nonfat Greek yogurt until it reaches a smooth, spreadable consistency.
Spread the Greek yogurt evenly onto both the top and bottom halves of the toasted brioche bun.
Layer the bottom bun with the butter lettuce leaf, the tomato slice, and the red onion rings.
Place the pan-seared chicken breast on top of the vegetables and finish the sandwich with the top bun.