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Soak the dried mushrooms in 250ml boiling water. Meanwhile, soften the shallots in the olive oil and a third of the butter for 10 mins. Add the squash and cook for a further 10 mins until softening, then add the garlic and herbs and cook for 2 mins more. Set aside.
In another pan, fry the fresh mushrooms in another third of butter until softened. Strain the porcini, reserving the soaking liquid, then roughly chop and add to the cooked mushrooms. Fry for 2 mins, then remove from the pan and set aside.
Melt the remaining butter in the same pan, stir in the flour for 2 mins, then gradually stir in the stock and mushroom soaking liquid. Bring to the boil, stirring, and cook for 2-3 mins until thickened. Combine the sauce with all the veg, then transfer the mixture to a 2-litre rectangular pie dish and cool.
Roll out the pastry on a floured surface until big enough to cover the pie dish. Cover the pie, trimming the excess pastry, then wrap well in cling film. Will freeze for up to 2 months.
To cook, defrost the pie thoroughly in the fridge overnight if frozen. Heat oven to 200C/180C fan/gas 6, glaze the pastry with a little egg and bake for 30-40 mins or until golden and hot through.