Put the gammon in a pan, add the onion, then pour over the Coke. Bring to the boil, reduce to a simmer, cover with a lid, and cook for 2ish hours. If your joint is larger or smaller, work out timings by going on an hour per kilo, remembering that it's going to get glazed in the oven later.
reheat the oven to 230C, When the ham's had its time in the coke take it out of the pan and let cool. Then remove the skin, leaving a thin layer of fat. Score the fat with a sharp knife to make large diamond shapes. Then carefully spread over the syrup. Gently pat the mustard and sugar onto the sticky fat. Cook in a foil-lined roasting tin for approximately 10/15 minutes or until the glaze is golden and bubbly.