Prepare and truss chicken. Put it in a heatproof casserole with peeled white onions, diced carrot and turnip, sliced celery and bay leaves. Heat chicken stock; add to casserole; cover and cook in a slow oven (325°-M2) for 1½ to 2 hours, or until the chicken is quite tender, basting occasionally with the stock. Season with salt and freshly ground black pepper. Cut chicken into serving pieces. Serve in casserole.