Brown the Beef
Begin by heating the rapeseed oil in a large flameproof casserole over medium heat. Brown the beef in batches, taking care not to overcrowd the pan, which ensures each piece is properly seared. Once browned, set the beef aside on a plate.
Cook the Vegetables and Bacon
Add the bacon lardons to the casserole and fry for 2-3 minutes, or until golden. Next, add the finely chopped onion, celery, carrot, garlic, and thyme. Allow the vegetables to soften by cooking gently for 6-8 minutes. While the vegetables cook, preheat the oven to 160°C (140°C fan), or gas mark 2.
Prepare the Sauce
Sprinkle the plain flour over the softened vegetables and cook for 1 minute. Pour in the Pedro Ximénez sherry, letting it bubble for a few minutes to reduce slightly. Then, add the beef stock and return the browned beef to the pan. Bring everything to a simmer, cover with a lid, and transfer the casserole to the oven. Let it cook for 3 hours.
Add the Celeriac
About 45 minutes before the cooking time is complete, remove the casserole from the oven and stir in the celeriac. Cover the dish again and return it to the oven for the remaining 45 minutes.
Shred the Beef and Cool the Filling
Once the casserole is done, remove it from the oven and place it on the hob. Use a slotted spoon to transfer the beef to a cutting board, then simmer the liquid until it reduces to a thick sauce. Shred the brisket into bite-sized pieces and return it to the casserole, seasoning well. Allow the filling to cool completely.
Assemble the Pie
Increase the oven temperature to 220°C (200°C fan), or gas mark 8. Transfer the cooled beef mixture into a pie dish. Lay the ready-rolled puff pastry sheets on a flat surface with the long edges facing you. Brush one sheet with a small amount of the beaten egg, then sprinkle evenly with the crumbled Stilton cheese. Place the second sheet of pastry over the top and gently press down to seal. Cut the pastry into 8 strips and twist each strip along its length. Arrange these twists over the pie, ensuring they fit snugly. Trim any excess pastry to match the size of the pie dish. Bake for 40 minutes or until the pastry is golden brown and well-risen.