Start the pastry: In a large bowl, whisk together the flour and salt. Add the cold butter cubes and toss to coat them in the flour. Using your fingertips, rub the butter into the flour until the mixture resembles coarse breadcrumbs with some pea-sized butter lumps remaining. You want these visible pieces of butter—they create the steam that makes the pastry flaky. Drizzle over the ice-cold water, a tablespoon at a time, and stir with a knife until the dough just begins to come together. It will look shaggy. Turn it out onto a lightly floured surface, bring it together into a disc, wrap in cling film, and chill for at least 30 minutes.

Brown the beef: Pat the beef cubes dry with kitchen paper (this is crucial for a good sear). Season them generously with salt and pepper. Heat 2 tablespoons of the oil in a large, heavy-based casserole dish or Dutch oven over a high heat until it shimmers. Working in batches so you don't overcrowd the pan, add the beef in a single layer. Let it sear without moving it for 2-3 minutes until a deep, dark brown crust forms. The sound of the sizzle will change as the crust sets. Turn the pieces and brown on all sides. Transfer the browned beef to a plate and set aside.

Sweat the vegetables: Reduce the heat to medium. Add the remaining tablespoon of oil to the pot. Add the onions, carrots, and celery. Cook, stirring occasionally, for 8-10 minutes until the onions are translucent and soft, and the raw smell has given way to a sweet, fragrant aroma. Add the minced garlic and cook for another minute until it becomes fragrant.

Build the base: Sprinkle the 3 tablespoons of flour over the softened vegetables. Stir constantly for 2 minutes until the flour is completely incorporated and has turned a pale golden colour, smelling slightly nutty. This is your roux. Slowly pour in the beef stock, whisking continuously to prevent lumps. The mixture will thicken as it comes to a simmer. Stir in the tomato purée, Worcestershire sauce, dried thyme, and bay leaf.

Braise the beef: Return the browned beef and any accumulated juices to the pot. Add the diced potatoes. Stir everything together, bring the liquid to a gentle simmer (you'll see small bubbles lazily breaking the surface), then reduce the heat to low. Cover the pot with a lid and let it cook gently for 1 hour 15 minutes to 1 hour 30 minutes. The beef should be fork-tender and the sauce will have darkened and become wonderfully rich. The smell of thyme and braised beef filling your kitchen is pure magic.

Finish the filling & cool: Remove the bay leaf and discard. Stir in the frozen peas and cook for just 2 minutes. Taste the filling and adjust the seasoning with salt and pepper. The texture should be thick and glossy, not watery. Remove the pot from the heat and allow the filling to cool completely—this is vital to stop the bottom of the pastry from becoming soggy as it bakes.

Assemble and bake: Preheat your oven to 200°C (180°C fan). Transfer the cooled filling to a 23cm round pie dish (or a 20cm x 30cm rectangular dish). On a lightly floured surface, roll out the chilled pastry to a thickness of about 3mm, and about 4cm larger than your dish. Gently lift the pastry over the filling. Crimp the edges to seal, and cut a few small steam vents in the top. Brush the entire surface with the egg wash. Bake for 30-35 minutes, until the pastry is a deep, golden brown and you can hear the filling bubbling gently inside. The crust should feel firm and sound hollow when tapped lightly.