Ingredients
4½ kg - 6kg Turkey or Cockerel (very large chicken) 
75g butter 
1 tbsp oil 
100g shallots, finely chopped 
60g dried porcini mushrooms, rehydrated in boiling water for half an hour and then finely chopped 
350g chestnut, portobello or button mushrooms (or a mix), finely chopped 
50ml Madeira 
50ml porcini water 
200g cooked chestnuts, finely chopped 
1 sprig of thyme 
Salt 
Pepper

Method
You can make the stuffing up to a day ahead and keep in the fridge. 

Remove your bird from the fridge.

Start by rehydrating your porcini mushrooms, for half an hour in boiling water. 

You can use your food processor to cut your mushrooms but only ever pulse slowly as otherwise you risk having mushed wet sludge.  The same goes for your chestnuts. 

Put 25g of butter and 1 tbsp olive oil in a large pan and gently cook the shallots for 3 minutes until soft and translucent.  Add both mushrooms and cook for 10 minutes until the juices have come out of the mushrooms and evaporated.  Put in the Madeira, porcini water, chestnuts and thyme and cook on a low heat for 5-10 minutes until the liquid has all evaporated again. Taste and season and then add the butter, mix and leave to cool.  Weigh. 

An hour before you want to eat take your bird of choice out of the fridge and allow to come to room temperature.  Preheat your oven to 180c. 

To stuff, you want to put the stuffing underneath the skin and over the breast.  To do this you need to very carefully separate the skin from the breast meat.  Take your hand and very carefully start to ease the skin away in slow sweeping motion until you get to the bottom.  If you find the middle part of the skin won’t easily come away just very carefully use a knife or kitchen scissors but do be careful, you really do not want any holes in the skin.  Carefully stuff the mushroom stuffing under the skin.  Take a sheet of kitchen paper and dry the skin then rub a tbsp of oil over and season.   

To roast, put the bird into a large roasting tin, stuffing side up, cover with foil and cook for 40 minutes per kg remembering to add the weight of the stuffing also.  Baste every 40 minutes or so, removing the foil for the last 40 minutes so it can go golden brown. 

Once cooked, remove from the oven and either use a thermometer to check the temperature (it should be around 70c) or, if you do not have one, just check the juices run clear.  Rest for 40 minutes before serving.