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Put the onion and 10 cloves in the base of the slow cooker then nestle in the gammon joint. Pour over the ginger beer then cover and cook on LOW for 7 hours until the gammon is tender, but still holding its shape. You can cool then chill the gammon at this stage if you prefer.
Heat the oven to 200C/180C fan/ gas 6. Carefully remove the skin from the gammon leaving a layer of fat behind. Score the fat in a diamond pattern with a sharp knife, making sure you don't cut into the meat, then stud the centre of each diamond with cloves.
Mix the mustard and ginger preserve in a bowl, spoon or brush over the gammon then bake for 20 mins until golden and sticky. If roasting from cold you will need to add another 20 mins to the cooking time.