First, make the stuffing. Melt half the butter in a heavy-based pan over a medium heat with the bay leaves and clove- studded garlic.
When the butter starts to foam, add the sliced onions, season with a good pinch of salt and stir well. Cook for about five minutes, until the onions are softened, but not browned. Add 50ml/1¾fl oz of the chicken stock and continue cooking.
After two minutes, add the sage. Cook until almost all of the liquid has disappeared.
Remove the garlic and bay leaves, spoon the onions and sage out onto a tray lined with kitchen paper to drain and allow the onions to cool.
When the onions are cool, stir in the chopped chestnuts.
In a mixing bowl, break down the sausage meat with a wooden spoon. Add the sage, onions and chestnuts and mix well. Season with salt and freshly ground black pepper. Cover and place in the fridge until you are ready to stuff the turkey legs.
To stuff the turkey legs, lay the turkey legs on a chopping board or clean work surface. With a sharp knife, trim off any fat or sinew. Then, using a rolling pin, beat the meat of the legs into two 1cm/½in thick rectangles and season with salt and freshly ground black pepper.
Place the stuffing down the centre of each leg and wrap the meat around so that the edges are overlapping by at least 6cm/2½in when the ends of the meat are folded in.
Brush two sheets of aluminium foil on one side with softened butter (the foil should be three times bigger than the meat). Wrap the legs tightly in the foil, with the buttered side on the inside. Transfer the turkey legs onto a baking tray and leave in the fridge to rest for an hour before cooking.
Season the turkey breasts with salt and tie together lightly with butchers' string to retain the shape. Wrap in a muslin cloth or tea towel and tie once again with the string.
Place the remaining chicken or turkey stock in a stockpot that is large enough to hold both breasts of turkey.
Bring to the boil with the garlic, thyme and two bay leaves. Carefully place the breast into the pan, making sure that they are covered with stock. Reduce the heat to a gentle simmer and cook, uncovered, for 25 minutes.
Preheat the oven to 170C/340F/Gas 3.
Take the stockpot off the heat and carefully remove the breasts using large tongs. Set the breasts aside. Unwrap the muslin cloth, but do not remove the string from the breasts. Remove the garlic and herbs from the stockpot and discard.
Return the pot to a medium heat and cook the stock until it reduces to about 1 litre/2 pints.
Meanwhile, remove the turkey legs from the fridge. Transfer the turkey legs to the preheated oven. The turkey legs should take about 2-2½ hours to cook. Turn the turkey legs regularly and check to see if they are cooked by inserting a skewer - the juices should run clear from the turkey with no blood and the skewer should be hot all the way through. Remove from the oven and allow to rest in a warm place for 15 minutes.
Weigh the breasts and make a note of the weight. Place the breasts in a heavy roasting tray using the bones from the legs and wings as a trivet. Rub 50g/1¾oz of butter under the skin of each breast and season the skin with salt and freshly ground black pepper. Cover loosely with aluminium foil and place in the base of the oven. Roast the meat for 35 minutes per kilogram, removing the tin foil after half an hour.
Check whether the breast is cooked by inserting a skewer into the thickest part of the meat - a clear liquid should flow out easily with no trace of blood. Remove the meat from the oven, cover loosely again with foil and rest in a warm place for at least half an hour before carving.
While the turkey legs and breasts are cooking, prepare and cook the vegetables.
For the potatoes, peel the potatoes and cut them into quarters. Place the potatoes in a pan and cover with cold water. Season with salt and bring to the boil over a high heat. Reduce to a simmer for five minutes; then drain in a colander and allow to cool a little, before drying off any excess moisture with kitchen paper.
Meanwhile, chop the carrots, parsnips, onions and salsify into chunks.
Add the rosemary, thyme, garlic and lemon to the vegetables. Season with salt and freshly ground black pepper and set aside.
When the turkey is removed from the oven, increase the oven temperature to 180C/350F/Gas 4. Heat half the duck fat in a large heavy roasting tray in the oven. When the fat seems to shimmer, carefully place the potatoes in the tray and season with salt. Transfer to the oven, and roast for about 45 minutes to 1 hour, turning regularly so that all the sides of the potatoes are crisp. When cooked remove from the oven and drain on kitchen paper. Sprinkle with a good pinch of salt.
Add the vegetables to the oven soon after the potatoes have gone in. Heat the remaining duck fat in a roasting tray in the preheated oven for five minutes. When the fat seems to simmer, carefully add the vegetables to the pan. Cook for 30-45 minutes, stirring the vegetables from time to time. Remove the lemon, garlic and herbs before serving.
To make the gravy, remove the bones from the tray used to roast the turkey. Place the tray on top of the hob over a medium flame, then stir in the cornflour, followed by the reduced chicken turkey stock. Pour into a saucepan and continue to cook over a medium heat for five minutes, skimming off any fat with a ladle.
Pass the gravy through a fine sieve into a gravy boat.
To serve, carve the turkey breasts and legs and serve with the potatoes, vegetables and bread sauce.