Preheat the oven to 200C / 180C fan / Gas Mark 6.
Heat the oil in a large pan and brown the beef mince over high heat, breaking it up as it cooks.
Add the sliced onions and cook for 5 minutes until they start to soften.
Sprinkle in the flour and stir well for 1 minute.
Gradually pour in the beef stock, stirring constantly to avoid lumps.
Add the Worcestershire sauce, salt and pepper. Simmer for 15 minutes until the mixture thickens.
Transfer the filling to an ovenproof dish and leave to cool slightly.
Roll out the pastry and drape over the dish, trimming the edges and crimping with a fork.
Cut a small slit in the top to let steam escape and brush with beaten egg.
Bake for 25-30 minutes until the pastry is golden brown. Serve hot with mash and vegetables.