Preheat the oven to 375 degrees Celsius.
Place the ham in a roasting pan and score it. Form diamond shapes by cutting through ⅓ inch into the fat.
In a saucepan, combine hoisin sauce, vinegar, soy sauce, and mustard. Bring to a soft boil while stirring constantly. Simmer for a few minutes.
Glaze the ham generously, making sure to get into the scored crevices.
Bake for 30 minutes. Re-glaze the ham and bake for another 30 minutes.
Re-glaze for the second time and cover it loosely with foil.
Bake for another hour until you achieve a sticky and golden glaze.
Allow the ham to sit before carving and serving.
