Heat the oil in a large skillet over medium high heat. Season the chicken with salt and pepper. Add to the pan and brown on both sides, about 3 minutes per side. Transfer to a plate and cover to keep warm.
Add the onion to the skillet and saute for 1 minute. Add the cider and chicken stock, and cook over high heat until the liquid has reduced by half. Add the chicken to the pan, cover, and reduce heat to low. Simmer for 5 minutes.
Remove the chicken breasts from the pan, and place on a warmed serving platter. Raise the heat to high and reduce the cider sauce until 1/4 c remains. Stir in the mustard and season with salt and pepper. Pour over the chicken and garnish with parsley.