Cover the porcini in 100ml boiling water and set aside. Heat 2 tbsp oil in a wide pan, add the onion and cook over a medium heat, stirring often for about 10 mins until soft. Push to one side of the pan, add the button mushrooms and thyme, fry until any water has evaporated and the mushrooms are soft and taking on some colour (about 4-6 mins). Scrape the onions and mushrooms into a bowl.

Heat the remaining oil in the pan. Toss the beef in the mustard powder, flour and some seasoning, then fry, in batches if needed, until each piece has a dark brown crust. Drain the porcini mushrooms, but keep the soaking liquid, and roughly chop them. Return all the beef, onions and mushrooms to the pan along with the porcini liquid. Add the wine and bubble for a few mins, scraping the bottom to release any tasty bits. Add the miso, soy and stock, season and cover the pan with a lid. Simmer for 2 hrs 30 mins-3 hrs until the beef is as tender as you’d like it. Stir every 15 mins or so, adding more water if it’s sticking to the bottom of the pan. Strain the stew through a sieve over a large pan or bowl and discard the thyme sprigs. Stir 100ml liquid back into the beef and transfer the rest to a jug – this will be your gravy. Cool the stew and gravy, then chill. You can do this up to three days before assembling the pie.

Mix the ingredients for the pastry, except the egg, in a food processor with a good pinch of salt. Slowly add water, a tablespoon at a time, until the pastry starts to clump together (ours needed 5 tbsp). Tip onto your surface and knead a few times to incorporate any flaky bits. Remove a third of the pastry and shape both pieces into discs, wrap in cling film or baking parchment and chill for at least 30 mins, or up to one day.

Heat oven to 200C/180C fan/gas 6 with a baking sheet heating up on the middle shelf. Roll out the larger piece of pastry, using a little flour if you need it, until about the thickness of a £1 coin. Lift over your rolling pin and use to line a metal or enamel cake tin or pie dish – we used a deep, loose-bottomed 20cm cake tin – leaving plenty of pastry hanging over the edges. Fill with the cooled stew. Brush around the inside edge of the pastry with egg then roll out the remaining pastry and use to cover the pie.

Squash or crimp the edges and roll the pastry back on itself to create a seal. Press the edge lightly with a fork, if you like, brush with egg and make a generous steam hole in the middle (we used the end of a large round piping nozzle to do this). Sprinkle with flaky sea salt. Bake for 45-50 mins until the pastry is deep golden brown, then leave to cool for 10 mins while you reheat the gravy. Remove the pie from the tin and serve with gravy, mash and veg.