Combine the beef, onion and beer in a large saucepan. Bing to the boil, reduce heat and simmer over low heat for 45 minutes or until the meat is tender.
Preheat the oven to 200C.
Season the beef with garlic, thyme, parsley, Worcestershire sauce, salt and pepper. Mix in the potatoes and mushrooms. Cover and simmer over medium heat for 10-15 minutes or until the potatoes are just tender. In a small bowl whisk together a small amount of the sauce and the flour and stir into the beef. Simmer until slightly thickened.
Trim one pastry sheet large enough to line the base and sides of a 23cm pie plate. Spoon the hot beef mixture into the pastry case and top with the remaining sheet of pastry, trimming to fit. Cut three slits in the top of the pie and press edges to seal.
Bake in oven about 35-40 minutes or until the top is golden brown.