Cut the stewing steak into 3cm/11⁄4in chunks. Put the seasoned flour into a plastic bag, add the steak chunks, close the top of the bag and shake until thoroughly coated with the flour.
Heat a cast-iron or other heavybased pan over a medium heat and add the oil. When hot, sauté the onions for about 3 minutes. Add the floured meat and cook, stirring occasionally, for about 10 minutes, until coloured all over. Pour over the stock or water and add the Worcestershire sauce. Bring to the boil, reduce the heat to a simmer and cook for about 2 hours.
Add the potato chunks and cook gently for a further hour.
Meanwhile, make the pastry (or use bought puff pastry). Rub the lard into the flour and the salt, or put into a food processor and whizz until the mixture resembles fine breadcrumbs. Add the water a little at a time and mix to a dough. Leave to rest for 20 minutes.
Check the meat – when ready, it will be soft and break apart when squeezed. The sauce should be rich and thick. Season as necessary.
Preheat the oven to 190°C, 375°F, Gas 5. Fill an ovenproof pie dish with the meat mixture until three-quarters full. Roll out the pastry until 3cm/11⁄4in thick, and cut into a shape large enough to cover the top of the dish.
Beat the egg and milk together and brush over the rim of the dish. Place the pastry over the top, pressing the edges down firmly on the dish to seal it. Brush the pastry with the egg mix, then cut small holes in the top to allow steam to escape. Bake for 40 minutes. (Cover the edges of the pastry with some foil if they become too brown.)