Preheat the oven to 160°C/Fan 140°C/Gas 3. Heat the oil in a heatproof casserole dish. Add the beef and cook, stirring, for 2–3 minutes, until browned. Remove the meat from the casserole dish, season and set aside.
Add the leeks, onion and carrots to the pan, adding a little more oil if necessary. Cook over low heat for about 3 minutes, until softened. Add the mushrooms, bacon and thyme and cook for a further 2–3 minutes. Season well. Add the garlic and cook for 1 minute.
Return the beef to the casserole dish, add the flour, stir to coat the meat in the flour and cook for 2–3 minutes. Pour in the Guinness and Worcestershire sauce. Add the bay leaf and parsley and pour in sufficient cold water to just cover. Stir to mix, cover with a lid and bake in the preheated oven for about 1½ hours.
Remove the casserole dish from the oven and increase the oven temperature to 200°C/Fan 180°C/Gas 6.
Transfer the beef mixture to a 30 x 20-cm baking dish. Unroll the pastry and use to cover the pie filling. It should look rustic, so fold over the edges and crimp roughly with your fingers. Using a sharp knife, and starting at the top edge, make lengthwise slits on the diagonal, in stripes about 1⁄2 cm apart, all the way across.
Brush with melted butter or milk and bake in the preheated oven for about 25–30 minutes, until golden. Serve immediately with mashed potatoes and peas on the side.