Place the beef, onion and ale in a large saucepan. Simmer over low heat until the meat is tender, about 30 minutes.
Preheat the oven to 200 C / Gas mark 6.
Season the beef with garlic, thyme, parsley, Worcestershire sauce, salt and pepper. Mix in the potatoes and mushrooms. Cover and simmer over medium heat until potatoes are just tender enough to pierce with a fork, 10 to 15 minutes. Whisk a small amount of the sauce together with the flour in a small bowl, and stir into the beef. Simmer until slightly thickened.
Fit pastry into the bottom and up the sides of a pie dish. Spoon the hot beef mixture into the dish and top with the remaining pastry. Cut slits in the top to vent steam, and crimp the edges to seal them together.
Bake in preheated oven until the pastry is golden brown and gravy is bubbling, 35 to 40 minutes.
This pie freezes well - simply assemble the pie and freeze before baking. Once you're ready to bake, simply throw it in the oven straight from the freezer.