The pastry needs to be made in advance, so you can leave it to rest in the fridge for a couple of hours ? this will make it much, much easier to roll out and manipulate. To start, rub the margarine and vegetable fat into the flour and salt until it starts to look like breadcrumbs. Then add the cold water and bring together into a smooth, elastic dough ? I used the dough hook attachments on my hand mixer, but you could use an ordinary mixer or knead it with your hands. It will take a bit longer to come together than other pastries, so don?t panic. Once the dough is even and nicely springy, wrap it in clingfilm and pop in the fridge for three hours.
Start on the filling, leaving enough time for it to cool before you assemble the pies. Slice the beef and mushrooms into small pieces (the pies are quite small so you can?t have big chunks in there). Melt a generous scoop of butter in a pan and add the chopped mushrooms and a good sprinkling of white pepper, stirring until the mushrooms start to darken in colour, then add the beef, Balsamic Onion Relish, a good sprinkle of thyme and the red wine. Season well and leave it all to bubble on a medium heat until the liquid has almost completely disappeared ? you don?t want the filling to be too wet or it will make the pastry soggy. Leave it to cool.
An hour before you want to serve, preheat the oven to 200?C (180?C fan) and assemble the pies. Divide the dough into 6 equal pieces, and roll each one out in turn, trimming to create circles about 16cm in diameter. Pop a spoonful of filling onto each, fold the pastry over the top, and roll up the edge firmly to stop the filling escaping ? press with a fork to make sure the join is sealed. Crack the egg into a bowl and beat the yolk and the white together, then use a pastry brush to spread the egg wash over the top of each pasty. Bake in the oven for 45 minutes, until the pies are golden-brown on top and smelling gorgeous.