1. To make the pastry, stir the salt and thyme leaves into the flour in a large bowl or a food processor. Add the diced butter and pulse or mix with beaters until the butter lumps are about pea-sized; it will only take a few seconds. Mix in the grated cheddar, add the egg and mix until it comes together. Add the iced water if it looks dry.
2. Gather the pastry into a ball and divide in two: two thirds of the whole amount for the pie shell and a third for the top crust. The above proportions will give you the large portion about 360g in weight and the small 180g. Wrap each piece in cling film, flatten slightly and chill for at least 30 minutes.
3. In the meantime soak the porcini mushrooms in a bowl of boiling water; leave to cool. Cut the chicken into bite-sized pieces, slice or chop all the other vegetables and cut the bacon rashers in 2cm pieces.
4. Take the pastry out of the fridge and place the large piece on a lightly floured surface. Flatten it by beating with a rolling pin, then roll it out, dusting with more flour, to a thickness of a pound coin. Line a 23cm (9in) pie dish with the pastry, trim the top (the pastry will shrink a bit while baking so leave a tall edge) and prick all over with a fork.
5. Preheat the oven to 180C/350F/gas 4. Line the pie shell with foil or scrunched up parchment and pour in baking beans or lots of small coins. Bake in the lower part of the oven, blind for 15 minutes, remove the paper and weights/coins and bake for further 5 minutes. Remove from the oven and leave to cool.
6. Roll out the top crust into a coin-thick disc, large enough to cover the pie dish. Return it to the fridge to chill while you make the filling.
7. Heat the oil in a large skillet or casserole over high heat. Brown the chicken pieces on all sides. Add the bacon and cook for a few minutes, stirring. Scoop out the meat with a slotted spoon onto a side plate.
8. Add the butter to the pan and turn the heat down to medium. Add the onion, garlic and celery and cook until softened, 5-7 minutes. Drain the mushrooms reserving the soaking liquid and chop them roughly; add to the pan with onions and continue cooking for another 3-4 minutes.
9. Sprinkle the flour over the vegetables and stir it in. Slowly pour in the mushroom liquid, then the stock, whisking it in. Let it bubble, add the chicken and bacon back to the pan, stir it well and let it cook down and thicken to thick gravy consistency, stirring often. Season with salt and pepper to taste, stir in the cream and parsley and take it off the heat. Let the filling cool down at least a little before building the pie.
10. Preheat the oven again to 200C/400F/gas 6. Spread the remaining grated cheese at the bottom of the pie shell. Spoon in the filling. Brush the edges of the shell with beaten egg; carefully cover with the pastry lid and press it down around the edge to seal. Trim the excess pastry. Brush the top with the beaten egg, cut out slits to let the steam escape and bake in the lower part of the oven for 40 minutes.
11. Leave it in the turned off oven for at least 15 minutes before cutting and serving.