Put the joint in a pan covered with cold water, bring to a boil, then tip into a colander and drain. Put the gammon in a pan, skin-side down if it fits like that, add the onion, then pour over the Coke.
Bring to the boil, reduce to a good simmer, put the lid on, though not tightly, and cook for just under 2½ hours.
Preheat the oven to 240°C/gas mark 9/450ºF.
When the ham is cooked take it out of the pan and let cool a little for ease of handling. Remove the skin, leaving a thin layer of fat. Score the fat with a sharp knife to make fairly large diamond shapes, and stud each diamond with a clove. Carefully spread the treacle over the bark-budded skin, taking care not to dislodge the cloves. Gently pat the mustard and sugar onto the sticky fat. Cook in a foil-lined roasting tin for approximately 10 minutes or until the glaze is burnished and bubbly.
Should you want to do the braising stage in advance and then let the ham cool, clove and glaze it and give it 30-40 minutes, from room temperature, at 180°C/gas mark 4/350ºF, turning up the heat towards the end if you think it needs it.