Preheat the oven to 160°C.
Mix all ingredients together in a small bowl.
Place the onion in the base of a non-metallic or enamel roasting dish and sit lamb on top. Pour glaze over the lamb then pour 200ml water around the lamb. Cover and place in the oven to slow-cook for 31/2 hours. Remove cover and continue cooking lamb for a further 30 minutes until the meat is almost falling off the bone.
Place lamb on a warm serving platter and cover loosely with foil and a clean tea towel. Strain cooking juices into a saucepan and skim away any fat with a large spoon. Boil to reduce the sauce, then whisk in the thickening paste.
Cook until the sauce is syrupy and taste for seasoning. If tart, add salt to balance flavour, then pour into a sauce jug. Remove cover from lamb and scatter over pomegranate seeds and fresh mint.