To make the flavoured butter, place the porcini into a small bowl, cover with boiling water and leave to soak for 5 to 10 minutes. Meanwhile, finely slice the garlic and roughly tear the wild mushrooms, then place into a medium frying pan over a medium heat with the thyme leaves and a lug of olive oil and fry for 5 to 8 minutes, or until golden. Using a slotted spoon, transfer the porcini to a board. Roughly chop and add to the pan, then pour the porcini liquid through a fine sieve into the pan. Cook for a further 3 to 4 minutes, or until thickened and reduced, then season to taste and leave to cool.
Meanwhile, preheat the oven to 200ºC/400ºF/gas 6. Remove the chicken from the fridge and leave to come up to room temperature.
Place the flour into a bowl, then gradually pour in 1.2 litres of cold water, stirring continuously until combined. Use your hands to bring it together into a soft, elastic dough, then cover with a damp tea towel and put aside.
Once the mushroom mixture has cooled, transfer to a board with the parsley leaves and a few drops of truffle oil, then roughly chop together. Scrape into a bowl, then squidge and mix in the softened butter.
Carefully lift up and push your fingers or a spoon between the chicken breast and skin on both sides of the bird to create pockets for your flavoured butter – be careful not to rip the skin. Rub most of the butter onto the breast meat, then replace the skin and massage the remaining butter all over the chicken. Tie the legs together with a piece of string to secure.
Roll the dough into a rectangle, roughly 30cm in diameter and 1cm thick. Cut off one-third of the rectangle lengthways, then place the chicken onto the smaller piece. Cover the chicken with the remaining dough, pressing the edges of the dough pieces together so you end up with a sealed parcel. Place in a large roasting tray and pop in the hot oven for 1 hour 40 minutes – be specific with your timings as you won’t be able to check whether the chicken is cooked until you carve the crust open.
Remove to a board to rest for around 30 minutes, then break open the crust with a knife and discard (you don’t eat the crust). Check that the chicken is cooked through, then carve it up and serve with creamy mash and steamed greens.