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First make the stuffing. Pour the madeira or wine into a bowl, then crumble in the mushrooms. Fry the onions in the butter for 10 mins, until golden. Cool, then mix with the thyme leaves, the mushrooms and their soaking liquid, and all remaining ingredients, apart from 8 of the chestnuts and the bacon. Season well.
Set aside half of the stuffing. Line a greased 500g loaf tin with bacon. Pack the rest of the stuffing into the tin, then bring the rashers round over the top and secure in place with cocktail sticks. Use the reserved chestnuts to fill the spaces where the bacon meets. Chill until ready to cook. This will keep in the fridge uncooked for 2 days or can be frozen for up to a month.
Prepare the turkey. The night before, put the onion quarters, reserved lemon halves and thyme sprigs in the cavity between the legs. Pack the reserved stuffing into the neck end. Secure the neck skin with skewers and tie the legs together. Weigh the turkey. Calculate the cooking time at 40 mins per kilo, plus 20 mins.
Put a large sheet of extra-wide foil in a large roasting tin and put the turkey on top. Smear the breast with the butter, then grate over half of the nutmeg and season well. Cover with bacon, then pour over the glass of madeira or wine. Seal the foil well to make a parcel. Chill overnight.
On the day, take the turkey out of the fridge 1 hr before roasting. Heat oven to 190C/fan 170C/gas 5. Put the turkey in the oven; then, 90 mins before the end of cooking, remove foil and bacon, and drain off the juices from the tin to use in the gravy. To test whether the turkey is cooked, push a skewer into the thickest part of the thigh – the juices should run clear. If they are pinkish, cook for 15 mins more, then test again.
Transfer the turkey to a platter, cover with foil, then a couple of tea towels, and allow to rest for at least 30 mins before carving. Meanwhile, cook the stuffing loaf for 30 mins and reheat the bacon. Garnish with watercress and serve the stuffing loaf separately.