Chop the butter into cubes and crumble the beef tallow into the flour and salt. Rub with your fingers till a breadcrumb texture. Stir through the grated Parmesan. Whisk the egg with the water, add to the dry ingredients then bring together into a dough by hand. Tip into worktop and gently knead to bring together. Cut into 4, press into small discs and refrigerate wrapped in plastic wrap.
Mix all ingredients thoroughly and refrigerate for 15 minutes.
Flour your surface and roll out each piece of pastry to a few mm thick in an oval shape. Take a small handful of the minced beef mixture and lay on one half of the pastry (approx 125g). Mix your leftover egg white with a little milk and brush around the area at the edge of the meat. Fold your pastry over and press gently around the meat. Using a fork crump the edges. I use my pasta cutter to trim the edge neatly. With a sharp knife cut a small hole in the top and move to a baking tray. Do the same for all 4 bridies. Brush the surface with the egg and milk mixture and put in a hot oven at 180c fan forced for 30 minutes until golden and bubbling.
Remove to a wire rack to cool a little before cutting and eating!