3 medium firm sweet apples, cored and cut into 1/2-inch slices (We used Gala)
2 teaspoons chopped fresh rosemary plus additional for serving
Heat 2 teaspoons of oil in a large skillet over medium high. Sprinkle the top of the chicken evenly with 1/2 teaspoon kosher salt and 1/4 teaspoon pepper. Once the oil is hot and shimmering, add the chicken to the pan, top-side down. Let cook for 4 minutes, then flip and continue cooking until the meat is cooked through and reaches an internal temperature of 160 degrees, 3 to 4 additional minutes. Remove to a plate and cover with foil to keep warm.
In a small bowl or large measuring cup, stir together the apple cider and mustard. With a paper towel, carefully wipe the skillet clean. Add the remaining 2 teaspoons of oil to the pan over medium high. Once the oil is hot, add the apple slices, remaining 1/2 teaspoon salt and 1/4 teaspoon pepper, and rosemary. Cook for 5 minutes, until the apples are lightly tender and begin to turn golden. Return the chicken to the pan and pour the cider mixture over the top. Let cook for 4 to 5 minutes, until the liquid is reduced by half. Serve warm, sprinkled with additional chopped rosemary as desired.