To make the brine, heat 200ml of the water,
salt, honey or sugar and a couple of thyme
sprigs in a small pan. When the salt has
dissolved, stir together with the remaining
water. Place the turkey in a bowl and pour
over the brine. Cover and chill for 4-8 hrs.
For the stuffing, heat a knob of the butter in a frying pan. When melted, add the shallot and fry for a couple of mins until softened. Add the mushrooms and fry for 5 mins until they start to turn golden. Pour over the Madeira, add the chestnuts and the leaves from 1 thyme sprig. Cook for 10 mins more until all the liquid has evaporated. Drizzle with the truffle oil and leave to cool, then mix together with the remaining butter.
Remove the turkey from the brine and discard the liquid. Use kitchen paper to gently pat the meat until it’s dry all over, then use your fingers or a spoon to gently ease under the skin of the bird, making a pocket between its flesh and skin. Smear the truffle butter all over the flesh, following the contours of the breast. Brush a little more butter over the skin, then place in a roasting tin. Can chill at this stage up to 1 day ahead.
Heat oven to 200C/fan 180C/gas 6. Roast for 1 hr (about 40 mins per kg). Halfway through cooking, baste with the juices and loosely cover with foil if the skin is browning. Check that the turkey is done by piercing the thickest part with a skewer – the juices should run clear. Remove from the tin and leave to rest, covered loosely with foil, for 10 mins.
While the turkey is resting, make the gravy. Pour away any excess fat from the roasting tin, then place the tin over a medium heat on the hob. Tip in the shallot, then cook for a couple of mins, stirring often. When the shallot has softened, sprinkle over the flour and mix through. Slowly pour in the Madeira, scraping up any caramelised cooking juices. If the gravy starts to turn lumpy, quickly whisk out any lumps. Bring to the boil and simmer until reduced by half, then pour in the chicken stock. Simmer for 5-10 mins until lightly thickened. Strain before serving, if you like.