1. Season the beef with salt and pepper. Toss in the flour and shake off the excess.
2. Heat 2 tbsp of the oil in a large nonstick frying pan over medium-high heat. In batches, add the beef and cook, turning, for 5 minutes until browned. Transfer to a large saucepan.
3. Add the remaining 1 tbsp oil to the frying pan. Add the onion and cook over medium heat, stirring, until softened. Add the mushrooms and garlic and cook, stirring, about 5 minutes, until the mushrooms begin to brown. Transfer to the saucepan. Stir in the stock, ale, Worcestershire sauce, tomato paste, thyme, and bay leaf into the saucepan and bring to a boil. Reduce the heat to medium-low, cover, and simmer about 1½ hours, or until the meat is tender.
4. Transfer the meat and vegetables to the pie dish. Reserve 2/3 cup of the sauce, and pour the rest over the meat mixture. Let cool.
5. Preheat the oven to 400°F (200°C). Roll out the puff pastry on a floured surface to a thickness of 1/8 in (3mm). Cut an 11in (28cm) round. Cut ¾ in (2cm) strips from the trimmings. Brush the rim of the pie dish with water, and place the strips around it. Brush with water. Place the pastry round over the dish and press the pastry edges together to seal. Trim off the excess with a knife.
6. Crimp the pastry edge and use the pastry trimmings to make decorations, if desired. Brush the pastry with beaten egg, make a hole in the middle, and affix the decorations on the pastry, if using. Place the dish on a baking sheet. Bake for 25 minutes, or until the pastry is puffed and dark golden. Serve immediately, with the reheated reserved gravy passed on the side.