Preheat the oven to 200°C/Fan 180°C/Gas 6. Heat the oil in a wide ovenproof pan or casserole dish over a medium heat. Season the duck with salt and pepper and pan-fry on all sides until golden. Help yourself with a pair of tongs. After 10 minutes, remove the duck from the pan and set aside.
Add the garlic cloves, chestnuts and onion to the pan and fry for 10 minutes. Add the thyme and prunes and cook for a further 5 minutes. Return the duck to the pan and flambé with the sherry. To do this, slowly tilt the pan towards the flame until the sherry catches, or light with a match. Let the flames flare up then die down and then add 250ml water.
Put the pan in the oven and cook for 25 minutes, then switch off the heat and leave the duck in the oven for a further 20 minutes without opening the door. This should crisp up the skin and cook the duck to perfection as well as resting the meat.
Serve as a centrepiece with some good bread
or roasted new potatoes.