Put the gammon, onion and 100ml water in the slow cooker, cover with the lid and cook on low for 6 to 8 hours (see Notes), or until the it's thoroughly cooked. The longer the gammon is cooked, the more like pulled ham it will become.
Remove the lid, lift the pork out of the slow cooker and onto a board, and leave it to stand for 10 minutes, or until cool enough to handle.
Preheat the oven to 220C/200C fan/gas mark 7. Very carefully, snip off the string and slide a knife, in a horizontal sawing motion, between the ham rind and fat, then lift off the rind, leaving a thin layer of fat on the meat. (You may need to do this in strips.)
Lightly score the ham fat with a knife in a crisscross pattern. Line a small roasting tin with a piece of tin foil, large enough to come up the sides of the tin. Place the ham on top, fat-side up. If the ham is thinner one end than the other, prop the short end up with a ball of crumpled tin foil, so the fat is more horizontal – this will help it brown more evenly.
Mix the mustard, honey and sugar in a small bowl and spread it all over the ham fat. Bring the foil in towards the ham to create a bowl to catch any sticky juices. Bake the ham for 12 to 15 minutes, or until the fat is nicely browned.
Serve in slices, warm or cold – see Notes for serving suggestions.