In medium saucepan, place chicken breasts, parsley, thyme, celery, carrot and onion. Cover with cold water. Bring to boil, lower heat and simmer chicken breast 15 minutes. Chicken is done when it feels firm to touch and meat thermometer inserted in the center reaches 170F. Remove pan from heat, uncover and cool chicken in liquid for 30 minutes. Transfer chicken to cutting board and shred.