Season the chicken breasts with salt and black pepper. In a large skillet, melt the unsalted butter over medium-high heat until it begins to bubble gently.
Place the chicken breasts into the skillet, cooking them for 4 to 6 minutes on each side, until the meat is thoroughly cooked and no longer pink in the center. Remove the chicken from the skillet and set aside.
To the same skillet, add the apple cider and Dijon mustard. Stir constantly as the mixture simmers for 2 to 3 minutes, allowing the glaze to thicken slightly.
Return the cooked chicken to the skillet, ensuring each side is evenly coated in the cider glaze. Top the chicken with the chopped pecans, and garnish with freshly chopped parsley before serving.