Blitz the flour and diced butter in a food processor until breadcrumb texture. Keep the processor spinning and gradually incorporate the cold water. When it forms into a ball, turn out onto a sheet of cling film and leave in the fridge for 1-2 hours. Then slice into four and roll out on a moderately floured surface into thin circles. Approximately 25 cm across (you can use a saucepan lid to then cut into a perfect pasty round)
For the filling combine all the ingredients in a large bowl. Place 200 g of filling in the centre of the pastry round and then fold both sides to the middle. Pinch the pasty closed around the edges and then start right to left and using your finger and thumb to roll the pastry over on itself into a rope-like crimp. At the end fold over the pastry back onto itself. Place a knife hole in the top of the pasty and place on baking paper. Brush with an egg wash and bake for 45 minutes at 180˚C.