Place the ham in a large roasting dish. Add the onion, garlic, rosemary, apple juice and water. Cover tightly with foil and place in a preheated oven.
Place in the oven and roast for 20 minutes per 500g/1lb. A 4kg/8lb ham/gammon will need 2 hours 40 minutes.
When the ham is cooked, remove from the oven and turn the heat up to 200°C/390ºF.
In a small saucepan, combine all the glaze ingredients and allow to come to the boil. Cook until the sugar has dissolved and the cherries are soft. Remove the cinnamon and ginger then blend with an immersion blender.
Cut the skin off the gammon and cut a diamond pattern into the fat.
Pour the glaze over the gammon and place back in the oven to cook for another 20-30 minutes.
Allow the glaze to get sticky but check on it regularly as it can burn quickly. Glaze the ham once or twice while baking to allow the glaze to get thick and sticky. When the glaze is sticky and dark, remove from the oven.
Allow the ham to cool for 30 minutes then slice and serve.