01 - Set oven temperature to 320°F.
02 - Toss beef cubes in flour and shake off any excess.
03 - Heat 1 tbsp oil in a large ovenproof casserole over medium-high heat. Brown beef in batches, adding more oil if necessary. Remove browned beef and set aside.
04 - In the same casserole, add onion, carrot, and celery; cook for 5 minutes until softened.
05 - Stir in garlic and mushrooms; cook for 2 minutes.
06 - Add tomato paste and cook for 1 minute.
07 - Return beef to pan. Pour in beef stock, Worcestershire sauce, add thyme and bay leaf. Season with salt and pepper.
08 - Bring to a simmer, cover casserole, and transfer to the oven. Cook for 1.5 hours until beef is tender and sauce thickens.
09 - Remove bay leaf. Adjust seasoning to taste and let filling cool to room temperature.
10 - Raise oven temperature to 390°F.
11 - Spoon cooled filling into a 9.5 inch pie dish. Drape pastry over filling, trim excess, and crimp edges. Cut a small slit in the center for steam to escape.
12 - Brush pastry with beaten egg.
13 - Bake for 35 to 40 minutes until pastry is golden brown and crisp.
14 - Allow pie to rest for 10 minutes before serving.