Combine the flour and salt in a large mixing bowl.
Rub in the diced butter until the mixture resembles breadcrumbs.
In a separate bowl, beat together the egg yolk and water.
Stir in the egg mixture until a dough forms.
Knead the dough on a floured surface until smooth.
Wrap the pastry in cling film and refrigerate for at least 30 minutes.
Preheat your oven to 180°C (160°C fan)/350°F/gas mark 4.
Heat some olive oil in a frying pan and sauté the onions and garlic until softened.
Add the diced beef to the pan and fry until browned all over.
Mix in the diced potatoes, tomato puree, Worcestershire sauce, dried thyme, salt, and pepper until well combined.
Pour in the beef stock, bring to a boil, then simmer for 10-15 minutes until the mixture thickens.
Roll out the pastry on a floured surface to about 3mm thickness.
Use a large cookie cutter or pastry cutter to cut out rounds of pastry.
Spoon some filling into the centre of each round of pastry.
Brush the edges of the pastry with a little water and fold over to enclose the filling.
Press the edges of the pastry together using a fork to create a crimped edge.
Place the pies on a baking tray lined with baking paper.
Bake in the preheated oven for 20-25 minutes until the pastry turns golden brown and crispy.
