Cut the onion horizontally to give three evenly thick slices. Chop the carrot and celery into 3 or 4 pieces of about the same length.
Lay the vegetables in the slow cooker to form a trivet and then place the gammon on top.
Add the peppercorns, allspice, cinnamon and bay leaves around the gammon joint.
Pour in the cider and the water.
Place the lid on and cook on HIGH for 4 - 6 hours or on LOW for 6 - 8 hours.
Remove the ham and vegetables from the slow cooker and set aside to slightly cool for 10 to 15 minutes. (The braising liquid can be kept for use as a stock on another occasion)
Pre heat the oven to 200C / 180C Fan / 400F.
Use a small sharp knife to carefully remove the rind and some of the fat from the gammon joint. Leave an approximately 1cm / ⅜” thick layer of fat intact.
Whilst not cutting too deeply, lightly score the fat in a diamond pattern.
Place the cooked vegetables on a baking tray lined with foil and rest the gammon on top of the vegetables.
Mix the glaze ingredients together and spread half onto the scored fat.
Roast in the centre of the oven for 15 minutes before covering with the remainder of the glaze. Roast for a further 15 minutes.
Cover loosely with foil and set aside to rest for 15 minutes before carving.