In a deep-based frying pan, heat some olive oil and seal the steak on all sides. Add the beef stock to cover and Worcestershire sauce, cover and simmer gently for about 2 hours. Remove the beef and place on a tray. Add the sweated off onions and reduce by three quarters until you have a thick onion coating sauce. Return the beef and allow to cool.
In a bowl, sift the flour and add the salt. Cut the butter into cubes and add to flour, rub in with your fingers not mixing everything completely. It's important to see some chunks of butter. Make a well in the centre add two thirds of the water. Mix until if forms a dough, adding more water if needed. Wrap in cling film and rest in your fridge for an hour.
On a floured surface roll out the pastry until you have a rough circle 3 mm thick. You will still see parts of the butter in the pastry. Take a layer of potato and lay in one half of the pastry, season with salt and pepper and repeat with a layer of cheese, then lay on the meat and pour over some of the beef gravy, then repeat with the potato and cheese, remembering to season. Fold over the pastry, with your hands cupping the edge, cut around it to create a semi-circle. Take a corner of the edge and then fold over and pinch, repeat to form the crimp around the pasty.
Repeat to make 4 pasties. Brush with egg yolk and put into a pre-heated oven at 190°C for 45 minutes. Let it rest for 15 minutes before eating.