Place the gammon in a pan, skin side down if it fits like that, add the onion then pour over the cola.
Bring to the boil, reduce to a good simmer, put the lid on, though not tightly, and cook for 1 hour. Skim off any scum.
Meanwhile, preheat oven to 240C/gas 9.
Take the ham out and place on a lined roasting tray. Let the ham cool a little for ease of handling. Score the fat with a sharp knife to make diamond shapes, and stud each diamond with a clove. Then spread the treacle over the skin taking care not to dislodge the cloves. Then gently pat the mustard and sugar onto the sticky fat.
Cook for approximately 10 minutes or till the glaze is burnished and bubbly.