Preheat the oven to 200C/180C Fan/Gas 6.
For the filling, melt the butter in a frying pan. Fry the shallots and mushrooms for about 5 minutes, or until the shallots are soft and most of the liquid from the mushrooms has evaporated. Set aside, then pat dry using kitchen paper. Transfer to a bowl and add all the remaining filling ingredients. Season generously, mix and set aside.
To make the hot water crust pastry, mix the plain flour and strong white bread flour in a bowl then rub in the butter using your fingertips. In a saucepan, heat 200ml/7fl oz water, the lard and salt until the lard is melted. When it reaches boiling point, quickly pour it over the flours and mix with a wooden spoon until the mixture forms a ball.
When cool enough to handle, briefly knead the dough and set aside one third of the pastry for the pie lid. On a lightly floured work surface, roll out the rest of the pastry and use this to line an oval raised pie tin or a 20cm/8in springform cake tin. Roll out the pie lid.
Add the filling and brush the edges of the pastry with egg wash to help the lid stick. Cover the pie with the pastry lid. Crimp the edges to ensure the filling is fully sealed. Use any leftover pastry to make shapes to decorate the pie lid (Mat made antlers). Stick the shapes onto the lid using egg wash.
Make a hole in the lid to allow steam to escape while the pie cooks.
Bake for 30 minutes, then reduce the oven temperature to 160C/140C Fan/Gas 3 for a further 1½ hours.
Serve at room temperature or cold.