Place the pineapple juice, wholegrain mustard, brown sugar, and water into a large snap-lock bag or container.
Add the ham and massage the marinade over the surface of the ham until well coated.
Refrigerate overnight to allow the flavours to develop.
Preheat the oven to 180°C bake. Turn the heat down if you think the marinade is starting to colour up too much. Or cover loosely with tin foil.
Transfer the ham to an ovenproof baking dish.
Bake for 45–60 minutes, basting the ham with the marinade every 15 minutes.
The ham is ready when it is heated through and the glaze has become slightly sticky and caramelized.
Rest for 10 minutes before slicing if serving hot.
Serve warm or allow to cool completely and serve cold.
You could use a slow cooker instead and cook it lower and slower. This would leave the oven free to use for other things.