Preheat the oven to 150ºC/130ºC fan/Gas 2.
Sit an enamel plate or small trivet in the base of a deep ovenproof pan. The gammon should ﬁt into the pan tightly.
Pour in the orange juice and add enough water to cover the gammon with liquid. Bring to the boil, skim the surface, then cover with a lid and transfer to the oven.
Simmer gently for 20 minutes for every 500g (1lb 2oz). Remove from oven and leave to become cold in the liquid.
For the orange glaze, put the sugar and 100ml (31∕2fl oz) of water into a saucepan. Stir gently over a low heat until the sugar has dissolved, then bring to the boil.
Add the thin slices of orange to the syrup, then cover and simmer for 10-12 minutes until the orange slices are soft but still holding their shape.
Lift out the orange slices, then continue to boil the syrup over a high heat until it has reduced by about half and thickened. Remove from the heat and leave to cool. It will thicken further.
Line a baking tray or roasting tin with foil and sit the ham on top. Remove the skin from the ham and using a sharp knife, score the fat.Brush the fat with a little of the syrup.
Arrange the orange slices in a neat row over the top, then brush with more syrup.
Preheat the oven to 220ºC/200ºC fan/Gas 7.
Cover the lean meat sides of the gammon with foil to protect the ham, but leave the orange-covered top open. Roast in the oven for about 30 minutes or until the top is glazed and lightly browned, or brown with a blowtorch.
To make the salsa, mix all the ingredients together in a bowl and season well. Carve the ham and serve with the salsa alongside.
Preparing the ham ahead of time
The glazed ham can be cooked and glazed up to three days ahead of serving cold.
Recipes extracted from Mary Berry Simple Comforts by Mary Berry (BBC Books, £26)
Visit our Christmas recipes section for more festive ideas, including more from Mary Berry