Toss the steak and kidney pieces in flour. Melt some butter in a large saucepan or cast iron casserole dish and brown the meat 4-5 chunks at a time to avoid creating steam in the pan.
When all the pieces of meat are browned, return them all to the pan on a low heat and add the mushrooms and stock cube/paste/whatever with enough liquid to nearly cover the meat. Cook on a back burner for 2 hours until tender, stirring occasionally to make sure it's not sticking on bottom of pan.
For The Pastry:
Meanwhile, make up the shortcrust pastry. Either rub the fat into the flour with your fingers or pulse in the food processor until it resembles breadcrumbs. If you use a food processor (like me) you'll need to transfer it to a large mixing bowl to bind.
To bind, add the mustard and a few tablespoons or the water. Stir with a metal spoon to bring together in large lumps then draw into a ball with your hands.
Form the pastry into two flat-ish discs and wrap with plastic film. Chill in the fridge for 30 minutes and around this time preheat the oven to 180c / gas 4.
After the time is up, remove from the fridge and roll out on a floured surface. Use 3in and 4in circular cutters to cut out pastry bases and lids. Push into the shape of the 12 hole muffin tin.
Assembling The Pies:
When the meat filling is cooked spoon some into each of the pie cases leaving most of the gravy in the pan. Press pastry lids on top and press down around the edges with a fork.
Brush egg yolk on top of the pie lids to glaze; bake in the oven for 25 minutes or until the tops are golden. Allow to cool in the tins for a few minutes then gently prise out and serve.