Make cross-shaped incisions in the base of the chestnuts. Cook in a saucepan of simmering water for 20-25 minutes. Strain and allow them to cool to one side.
Preheat the oven to 190°C.
Season the inside of the cavity with salt and black pepper then stuff it with the cut onion and orange. Line a large roasting tray with a large trivet. Smother the turkey all over with the softened butter then season the outside generously. Plate on the trivet and cover the tray loosely with aluminium foil. Roast for 2½ hours.
Meanwhile, peel the chestnuts and reserve to one side.
Remove after 2½ hours and remove the aluminium foil; reserve it for later. Increase the oven to 220°C, then remove the turkey and trivet from the roasting tray.
Combine the chestnuts and onions in the base of the roasting tray. Sit the turkey on top of them and return the tray to the oven for roughly 20 minutes to let the turkey brown some more and cook until completely cooked and the juices run clear when a skewer is inserted.
Remove once nicely browned and cover really well with the aluminium foil to insulate. Allow the turkey to rest for 20-30 minutes before spooning the chestnuts and onions onto a serving platter. Sit the turkey on top and serve.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.