Preheat the oven to Gas Mark 5, 190°C, fan 170°C. For the stuffing Heat the oil in a small pan and fry the celery gently for 4-5 minutes or until it is softening slightly but still has some crunch. Soak the tomatoes in boiling water for 10-15 minutes to plump them, then chop small. Purée the chickpeas with the chestnuts and Madeira in a food processor. How smooth you make the purée will depend on how smooth or 'bitty' you like your stuffing. Add the celery and tomatoes, season to taste and use to stuff the turkey. For the turkey Rub the turkey all over with butter and sprinkle with sea salt. Place the stuffing inside the cavity. Place the onion halves in a large roasting tin and sit the turkey on top. Cover with kitchen foil, tucking under the tin to create a foil tent. Roast according to the instructions, but remove the foil 30 minutes before the end until golden. To test the meat is cooked, pierce the fattest part of the thigh with a skewer. The juices should run clear. Rest the turkey, covered loosely with foil, for at least 30 minutes. Cook's tip Leftover meat will stay juicer if you keep it on the bone. Wrap the turkey carcass in a clean damp tea towel and cling film then store in the fridge.