Make sure the butter and water are cold before making the pastry.
Finely dice the butter, add it to the flour and rub with your fingertips until the mixture resembles fine breadcrumbs. Make a well in the centre of the bowl and add most of the water and a pinch of salt.
Mix the dough without overworking it. You just want to bring the ingredients together.
Roll the mixture into a ball, flatten slightly and cover with cling film. Allow to rest in the fridge for at least 30 minutes. When you are ready to top the pie, roll the pastry out to a thickness of 5mm. Brush the edges of the pie dish with egg wash and place the pastry on top. Trim the edges and score a pattern in the top if desired.
Peel and dice the onion, the carrot and the celery.
Heat a large saucepan over a medium heat and add the oil. Add the mince and cook until browned. Add the flour and cook for a minute before adding the diced vegetables, the tomato purée, Worcestershire sauce, mustard and stock. Reduce to a simmer and cook for 90 minutes, adding a little water if the sauce gets too thick. You are looking for a thick, rich gravy. Pour into an ovenproof pie dish.
Preheat the oven to 180'C.
Beat the egg and brush on to edges of the pie dish. Carefully lift the pastry on to the dish and trim the edges. Brush the remaining egg on to the pastry and then bake in the oven for 25 minutes or until golden brown.
«Enrich the sauce by using herbs like thyme or rosemary and try adding garlic. Use red wine as well as beef stock when cooking to give more depth of flavour to this staple dish.»