In a large mixing bowl, combine the flour and salt.
Add the diced butter to the flour mixture and rub in until it resembles breadcrumbs.
In a separate bowl, beat the egg yolk and water together.
Add the egg mixture to the flour mixture and stir until it forms a dough.
Knead the dough on a floured surface until smooth.
Wrap the pastry in cling film and chill for at least 30 minutes.
Preheat the oven to 180°C (160°C fan)/350°F/gas mark 4.
Heat the olive oil in a frying pan and sauté the onions and garlic for 2-3 minutes until softened.
Add the diced beef to the pan and fry until browned all over.
Add the diced potatoes, tomato puree, Worcestershire sauce, dried thyme, salt and pepper, and stir well to combine.
Pour in the beef stock, bring to the boil, then reduce the heat and simmer for 10-15 minutes until the liquid has reduced and the mixture is thickened.
Roll out the pastry on a floured surface to a thickness of about 3mm.
Cut out rounds of pastry using a large cookie cutter or pastry cutter.
Place a spoonful of filling in the centre of each round of pastry.
Brush the edges of the pastry with a little water and fold over to enclose the filling.
Use a fork to press the edges of the pastry together and create a crimped edge.
Place the pies on a baking tray lined with baking paper.
Bake in the preheated oven for 20-25 minutes, or until the pastry is golden brown and crisp.