01 - Preheat the oven to 320°F.
02 - Toss the beef cubes with plain flour, shaking off any excess.
03 - Heat 1 tbsp vegetable oil in a large ovenproof casserole over medium-high heat. Brown beef in batches, adding extra oil if necessary. Remove and set aside.
04 - In the same casserole, add onion, carrots, and celery. Sauté for 5 minutes until vegetables start to soften.
05 - Add garlic and mushrooms, cooking for 2 minutes.
06 - Stir in tomato paste and cook for 1 minute.
07 - Return beef to the pan. Pour in beef stock, Worcestershire sauce, sprinkle thyme, and add bay leaf. Season with salt and pepper.
08 - Bring to a simmer on the stove, cover with a lid, then transfer to the oven. Cook for 1.5 hours until beef is tender and sauce has thickened.
09 - Remove the bay leaf. Taste and adjust seasoning if necessary. Allow filling to cool to room temperature.
10 - Raise the oven temperature to 390°F.
11 - Spoon the cooled filling into a 9.5-inch pie dish.
12 - Drape pastry over the filling, trim excess, and crimp the edges. Cut a small slit in the center for steam to escape.
13 - Brush the pastry surface with beaten egg.
14 - Bake for 35 to 40 minutes until pastry is golden brown and crisp.
15 - Allow pie to rest for 10 minutes before serving.