Heat the oil in a deep frying pan and fry the beef mince for 4-5 minutes, breaking it up with a wooden spoon as it browns
Add the onion and cook for 2-3 minutes, then stir in the tomato purée and cook for 2-3 more minutes
Stir in the flour and cook for a further minute, then add the chopped mushrooms, the stout or beef stock and a couple of dashes of Worcestershire sauce
Bring to the boil, then reduce the heat, cover the pan with a lid and leave to simmer for 20 minutes
Set aside and leave to cool, then turn the meat mixture into a one litre pie dish
Roll out the pastry on a floured work surface until it is slightly larger than the pie dish
Gently drape the pastry over the dish, pressing firmly onto the edges
Trim, then shape the edges into a fluted shape
Cut some leaf shapes out of the pastry trimmings and decorate the top of the pie, sticking them to the pastry with the beaten egg yolk
Make three or four slits in the pastry to allow the steam to escape, then brush the pie with the rest of the beaten egg yolk and bake in the oven for 20-25 minutes, or until golden-brown
