For the crust:Lightly flour a clean work surface. Roll out 2 sheets of the puff pastry quite thinly. Cut out 12 discs - large enough to be pressed into the bases of the muffin tin with a little excess to attach the lids. With the other two sheets, keep them slightly thicker. Cut out 12 smaller discs that will become the pie lids. Line each cup of the muffin tin with the larger puff pastry discs. Place the 12 lids on a lined baking tray and place all of the pastry in the fridge.
For the filling: Heat a light drizzle of olive oil in a large frying pan.Fry off the mince until golden brown and set aside. In the same pan add a fresh drizzle of olive and sauté the mushrooms until golden brown and they have released all of their liquid. Season lightly and set aside with the mince. Add a fresh drizzle of olive oil to the pan and sauté the onion and celery until soft.Add the garlic, rosemary and dried herbs. Cook for a minute. Add the tomato paste and cook until it turns a dark brick red colour. Pour in the Worcestershire sauce and beef stock. Add the mince and mushrooms back to the pan. Bring everything to a simmer and cook until all the flavours have melded together. Taste to adjust seasoning. Remove from the heat and allow the filling to cool.
To assemble the pies:Preheat oven to 180˚C. Fill the base of the pies with the mushroom mixture. Rub the edges with a little water to help the lids stick. Cover with the pastry lids. Pinch the edges of the pastry together to seal. Pinch them at even intervals to create a cute pattern around each pie. Cut out a little steam hole in the top of each pie. Brush them with egg wash and sprinkle with dried herbs.
Bake for 25-30 minutes until golden brown and the pastry is cooked through. Allow to rest for 10 minutes before removing from the tins and tucking in!