Place the dried mushrooms in a bowl and pour over the just-boiled water. Set aside for 15 minutes.
Meanwhile, add 4 tablespoons olive oil to a large cast-iron pot and add the onions. Cook gently for 15 minutes, to release the sweetness, then turn up the heat and allow them to start to caramelise - this should only take a further 5 minutes.
Add the garlic and cook for 2 minutes until fragrant then stir in the flour. Cook the flour for about a minute then slowly add the Guinness, stirring well after each addition - this will ensure your sauce doesn’t have any lumps.
Add the veg stock cube, thyme, rosemary, bay leaves, tomato puree, marmite, and the rehydrated mushrooms with their soaking liquor. Stir to combine everything then turn the heat down to a simmer.
In a large frying pan heat 2 tablespoons of olive oil and fry the sliced mushrooms until golden. You may have to do this in batches as there are so many mushrooms. Add the mushrooms to the sauce and stir to combine. Turn off the heat and allow the mixture to cool completely.
Take a pie dish and unfold your pre-rolled pastry. Use the pie dish as a template to cut a circle for the lid of the pie. Set the pastry aside in the fridge keeping any offcuts to use later. Spoon the mushroom mixture into the pie dish and preheat the oven to 220 C / 200 C fan.
Using your offcuts, line the lip of the pie dish with pastry - this will make it easier for your lid to attach. Place your lid on top of the pie. Crimp the edges of the pastry to secure it to the dish. Cut a hole in the centre of the pie and if you wish, use the remaining offcuts to make some pastry decorations. Place your pie into the freezer for 15 minutes then place into the preheated oven and cook for 30 - 40 minutes until the pastry is risen and golden and the filling is piping hot throughout. Serve with wilted greens and mashed potatoes for soaking up all the lovely gravy!