Combine the chicken breasts, stock, sherry or wine, and the bouquet garni in a large flameproof casserole or deep frying pan. Bring to the boil. Lower heat, cover; poach chicken for about 20 minutes, skimming off foam that forms, as necessary.
Meanwhile, cut all the vegetables into bite-sized pieces.
After chicken has been poaching for 20 minutes, add all the vegetables to pan. Cover; cook until vegetables are tender and chicken juices run clear when breasts are pierced with the tip of a knife, about 6 minutes. Discard bouquet garni.
Spoon the pan juices and vegetables into large, shallow serving bowls. Top with chicken, whole or sliced, as you prefer. Sprinkle with parsley and serve.