For the pithiviers, place the legs, skin side down into a large bowl. Sprinkle over the sea salt, crushed black peppercorns, sliced garlic, thyme leaves, bay leaf and crushed juniper berry. Lightly mix together, then cover the bowl and marinate in the fridge for 6 hours. This is a light curing to slightly season and flavour.
After 6 hours, rinse the marinade off the leg meat and pat dry with a clean kitchen towel.
Place 600g/1lb 5oz of the duck fat into a small saucepan and heat to 80-90C/174-194F (check the temperature using a food thermometer) over a low heat. Add the pheasant legs and bring back to the same temperature. Gently cook for 1 hour and 30 minutes, checking often to make sure the temperature does not go above 90C/194F (the fat should be kept just under simmering point). This gentle cooking will slowly break down the firm collagen within the tissue of the muscle, changing the texture of the meat so it is melting. When the legs are cooked through, set aside and allow to cool in the fat.
Season the pheasant breasts with a little sea salt and freshly ground black pepper.
Heat a frying pan until medium-hot, add the tablespoon of duck fat and fry the pheasant breasts, flesh side down, for 3-4 minutes, until golden-brown. Turn the breasts over, increase the heat and fry for 4-6 more minutes, or until the skin is golden-brown and crisp and the breasts are medium-rare. Remove from the heat and set the breasts aside to rest for a few minutes before finely chopping into small cubes. 6. Return the pan used to cook the pheasant breasts to the heat, add the onions and fry for about one minute, then add the thyme, sage, bay leaf, and juniper berries and cook for 3-4 minutes, or until the onions are softened and light golden-brown.
Increase the heat to high, then add the mushrooms and fry for 2-3 minutes, or until golden-brown and any excess moisture in the pan has evaporated.
Add the crumbled cooked chestnuts and mix well. Transfer the mixture to a large mixing bowl and stir in the dried cranberries and blueberries. Season, to taste, with sea salt and freshly ground black pepper and stir in the chopped pheasant breasts.
De-glaze the pan with the water and stir into the pheasant mixture..
Remove the cooled confit duck leg from the duck fat and place into a hot, dry frying pan. Turn the heat up and cook for 3-4 minutes on all sides, or until golden-brown and crisp all over. Remove from the pan and cool slightly before flaking the meat off the bone. Roughly chop the leg meat, then stir into the pheasant breast mixture. Cover the bowl with cling film and chill in the fridge until needed.
Meanwhile, roll out the puff pastry dough until it is 2mm thick. Cut out eight 10cm/4in discs and eight 14cm/5½in discs and place onto trays lined with parchment paper. Chill the pastry in the fridge for about five minutes.
Grease a small bowl or cup that is no more than 8cm/3in in diameter and pack about 60g/2½oz of the filling into the base, pressing down firmly.
Turn the filling out from the bowl or cup onto the centre of the smaller pastry disc. Brush the edges of the pastry disc with some of the beaten egg, then drape the larger pastry disc over the filling. Press around the filling with your fingers to seal, making sure there are no air pockets. Repeat the process with the remaining pastry and filling, placing each finished pithivier onto a tray lined with parchment paper. Chill in the fridge for 10-15 minutes.
Remove the pithiviers from the fridge and trim off any excess pastry using a 10cm/4in pastry cutter. Brush the pithiviers with the remaining beaten egg, then score the top with the back of a knife, creating half-moon patterns across the pastry. Chill in the fridge until ready to cook.
Preheat the oven to 200C/400F/Gas 6. Place a baking sheet in the oven to heat up.
For the sauce, take the reserved pheasant carcass and chop into 3cm/1¼in pieces.
Heat a large frying pan until hot, add the duck fat, pheasant carcass and juniper berries and fry for 4-5 minutes, or until the carcass is golden-brown all over. Transfer the pieces to a bowl.
Return the pan to the heat and fry the onion and celery for 3-4 minutes, or until light golden-brown. Add the mushrooms and cook for 1-2 minutes, then return the carcass pieces to the pan.
Pour in the Madeira , bring to the boil, add the port and reduce in volume by half.
Add the cold water to the pan and bring to the boil, then reduce the heat and simmer for 20 minutes.The cold water will trap any impurities and force them to the surface as it comes to the boil. Season, to taste, with salt and freshly ground black pepper. 20 minutes is just long enough to extract all the flavours from the carcass - any longer than this and it will start to stew and taste old.
Pass the mixture through a fine sieve into a clean pan, pressing down well to extract as much sauce as possible. Bring the sieved sauce to a simmer, then add the diluted arrowroot to the pan and stir. Cook for 1-2 minutes, until the sauce thickens enough to coat the back of a spoon.
Slide the pithiviers onto the hot baking sheet and bake for 15-20 minutes, until the pastry is golden-brown and crisp.
For the garnish, heat a frying pan over a medium heat and lightly dry fry the prunes, raisins and walnuts for 2-3 minutes, shaking the pan frequently to prevent burning, until heated through.
To serve, place a pithivier onto the centre of each serving plate. Using a small teaspoon, make a small hole in the centre of the pithivier. Spoon some of the sauce into the hole of each pithivier, and around the plate. Scatter the warm prunes, golden raisins and walnuts around.