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.
Cut the steak into 1 inch pieces and roll in the flour, salt and pepper. Peel and dice the onion, the carrot and the celery. Dice the kidneys.
Heat a large saucepan over a medium heat and add the sunflower oil. Add the steak, brown on all sides and then remove from the pan and add the kidneys. Once the kidneys are browned on all sides, return the steak to the pan and add the diced vegetables, the tomato purée, Worcestershire sauce, the mustard and the thyme. Cook for 2 minutes and then add the stock. Cook over a medium to low heat for 90 minutes. The meat should be tender and starting to fall apart - the cooking time will be less if the meat has been finely diced. Add a little water to the pan as it cooks if the sauce becomes too thick but you are look for a rich gravy.
Preheat the oven to 180'C.
Beat the egg and brush on to the sides of your pie dish. Lift the pastry on to dish, trim away the excess and crimp the sides with your fingers. Transfer to the oven and bake for 25 minutes or until golden brown on top.
«Remove any fat and sinew from the kidneys when you dice them.»