In a large pot, melt butter and add flour on low heat, stirring continuously.
Add beef stock, onions, mushrooms and chopped rosemary
Add 2 cups of braising liquid (or meat juices) from the leftover brisket cook, including the layer of congealed fat
Add Guinness and cubed brisket and simmer on medium heat for 3-4 hours or until the liquid reduces and thickens to a gravy consistency and the brisket starts to pull apart. Stir occasionally.
Cut the puff pastry to fit the pie trays (allowing approx 1cm extra so that the pastry covers the lip of the pie tray)
Spray pie trays with cooking spray oil and line the puff pastry around the base of the tray.
Fill the pie bases with brisket gravy then cover with puff pastry
Press around the outter edges with your thumb then using a fork press down on the outer edges of the pastry to seal the bottom and top layers of pastry making a pie.
Puncture tops of pies with fork holes to help with cooking
Combine egg and mil and baste the egg wash over the top of the pastry. Add a sprinkle of grated cheese on top
Place pies in a preheated oven at 200 degrees Celcius (400F) and cook for 20-25 mins or until golden brown.
Allow to rest for 5 mins before serving