Put the onion, carrot, celery, bay leaves and peppercorns into the slow cooker. Remove all packaging from the gammon and place in the slow cooker. Add just enough cold water to cover the gammon but take care not to overfill your slow cooker.
Cover with the lid and cook on high for 3-4 hours. Remove the cooked gammon from the cooking liquid using a large, slotted spoon and allow to drain. You can either discard the cooking liquid and vegetables; or strain the liquid, discard the vegetables and save to use as a stock for future use. Preheat the oven to 200°C.
Place the well-drained gammon joint in an ovenproof dish or roasting tin. In a small bowl, mix the honey, mustard and Chinese five spice powder together until smooth. Spoon half of the glaze all over the top and sides of the gammon, spreading it over with the back of a spoon.
Place the glazed gammon in the preheated oven for 10 minutes until the glaze becomes sticky. Remove from the oven and spread the remaining glaze all over the gammon, then return to the oven for a further 10 minutes until the glaze is sticky and slightly caramelised.
Remove from the oven and allow to rest for 5 minutes before slicing. This will make it easier to slice. Serve hot or cold with an accompaniment of choice.