In a large glass bowl, combine hoisin sauce, soy sauce, sugar, sherry, ginger, and mustard, mixing well. Add chicken, turn to coat, and marinate for at least 2 hours, turning often and allowing mixture to pour inside of chicken.
After marinating, reserve marinade and place chicken in a roasting pan in a preheated 350 degree oven for 1-1/2 hours, until cooked. Remove from oven, and cut into serving pieces.
Heat reserved marinate in a saucepan, and pour over chicken pieces. Garnish with chopped scallions.