Unwrap the gammon, but leave the strings tied. Place the gammon joint in the bowl of your slow cooker. Roughly chop the onions, carrots and apple and place around the ham. They’re just used to flavour the cooking liquid for the gammon.
Pour the cider (or apple juice) and chicken stock into the slow cooker and cook on high for about 3 hours or on low for 5-6 hours. *See notes
Mix the maple syrup, honey (if using), and mustard and mixed spice in a small sauce bowl. Whisk well. Preheat the oven to 200ºC.
Remove the gammon from the slow cooker and place on a baking tray lined with foil. Remove the string from around the ham and use a sharp paring knife to score the skin. Pour half the glaze over the cooked gammon and place in the middle of the preheated oven for 15 minutes. Remove the tray and pour a little more of the glaze on the ham, and place it back into the oven for a further 10 minutes.
Once the skin is crispy and well glazed, remove it from the oven and cover lightly with some foil while you let the ham rest for 15-30 minutes before serving.
Slice the gammon and serve with more mustard and drizzle with the last of the glaze.