To begin, preheat your oven to 200°C (390°F). In a large, heavy-bottomed skillet, heat the vegetable oil over medium heat. Add the finely chopped onion and garlic, sautéing until softened and translucent, about 3-4 minutes. This gentle cooking releases their sweetness and ensures the onion flavour melds seamlessly into the filling.

Next, add the ground beef to the skillet. Using a wooden spoon, break the meat into small pieces as it browns, stirring occasionally. Allow it to cook evenly for about 6-8 minutes until fully browned. Be sure to drain any excess fat to keep the filling from becoming too greasy.

Once the beef is browned, sprinkle in the all-purpose flour and stir it into the mixture. The flour will act as a thickening agent, ensuring your filling has the right consistency. Cook for 2-3 minutes to remove the raw flour taste, stirring constantly.

Slowly pour in the beef stock, stirring continuously to avoid lumps. Add Worcestershire sauce, tomato paste, and season with pepper and salt to taste. If you’re adding mushrooms, this is the time to mix them in. Simmer the filling over low heat for about 10 minutes, allowing it to thicken and reduce. Stir occasionally to prevent sticking. The filling should be rich and slightly saucy without being too liquid.

While the filling simmers, prepare your pie crusts. Lightly grease 4 individual pie tins or a large 9-inch pie dish. Line each with shortcrust pastry, pressing the dough firmly into the edges. Trim any excess dough hanging over the sides, and prick the base with a fork to prevent puffing during baking.

Remove the meat filling from heat and let it cool slightly. This step is crucial, as filling the pies with hot mixture can cause the pastry to become soggy. Once slightly cooled, divide the filling evenly among the pie tins or pour it into the large pie dish.

Cover each pie with a sheet of puff pastry. Press down the edges to seal the top and bottom layers of pastry together, crimping with a fork for a decorative finish. Trim any excess pastry. Using a sharp knife, cut small slits in the top of each pie to allow steam to escape during baking.

Brush the tops of the pies with the beaten egg for a glossy, golden finish. Bake in the preheated oven for 20-25 minutes, or until the pastry is golden brown and crisp. Allow the pies to cool for a few minutes before serving.