1) Place the lamb, onion, the split chilli, bay leaves and garlic into a pot large enough to hold everything comfortably. Cover with cold water, add two teaspoons of salt, then bring to the boil and simmer for 45 minutes, ideally with a lid on.
2) Preheat the oven to 180C/Gas 4.
3) Heat up a roasting dish at the same time with the butter and olive oil in it. Remove the lamb from the poaching liquor and place it in the dish. Put it in the oven and roast for 30 minutes.
4) Mix the sweetcorn and carrots with the remaining chilli and season with salt. Turn the lamb over and place the vegetables around it then return it to the oven.
5) Cook the lamb for a further 15 minutes then remove it from the dish and cover with foil to rest for 15 minutes.
6) Mix the vegetables with the roasting juices and cook a further 10 minutes, then transfer the vegetables and all of the pan juices to a serving dish.
To serve, slice the meat off the bone and serve with the vegetables. Mashed potato is an excellent accompaniment to this, especially if you mix hot English mustard paste and cream into the mash.