Preheat your oven to a robust 200°C (400°F/Gas Mark 6) to ensure a beautifully crisp pastry. Heat the vegetable oil in a deep frying pan over medium-high heat. Add the minced beef and cook it for 4 to 5 minutes, breaking it up with a wooden spoon as it browns evenly. This step locks in the meat’s rich flavor and gives a lovely texture. Stir in the chopped onion and let it cook gently for 2 to 3 minutes until softened and translucent. Next, mix in the tomato purée and allow it to cook for another 2 to 3 minutes; this deepens the savory base and balances the richness. Sprinkle the plain flour over the mixture, stirring continuously for around a minute to cook out the raw taste and help thicken the filling. Add the chopped mushrooms, then pour in the beef stock (or stout, if using) and a couple of dashes of Worcestershire sauce. Bring the mixture to a gentle boil, then reduce the heat to low and cover the pan with a lid. Let it simmer for 20 minutes, allowing flavors to meld and the filling to thicken beautifully. Once cooked, remove from heat and let the hearty filling cool slightly before transferring it into a 1-litre pie dish. Cooling helps prevent the pastry from becoming soggy. On a floured work surface, roll out the shortcrust pastry until it’s slightly larger than your pie dish. Carefully drape the pastry over the dish, pressing firmly around the edges for a neat seal. Trim off the excess dough, then shape the edges into a pretty fluted pattern. From the pastry trimmings, cut out charming leaf shapes and stick them gently atop the pie using some of the beaten egg yolk as adhesive. This adds a decorative flair that makes your pie shine. Make three or four small slits in the pastry top to allow steam to escape during baking, preventing sogginess or pastry from bursting. Brush the entire pastry surface generously with the remaining beaten egg yolk to give it a glossy, golden finish once baked. Place the pie in the preheated oven and bake for 20 to 25 minutes, until the crust is golden-brown and irresistibly crisp. Remove the pie from the oven, let it rest for a few minutes to set, then slice into wedges and serve warm with your favourite sides.