Make the pastry. In a large bowl, rub the cold butter into the flour and salt until the mixture resembles coarse breadcrumbs — you should still see small, pea-sized pieces of butter. Gradually add the ice-cold water, mixing with a knife until the dough just comes together. Wrap in cling film and chill for at least 30 minutes.

Cook the chicken. Season the chicken thighs generously. Heat the olive oil in a large, heavy-based pan over a medium-high heat. Sear the chicken for 4-5 minutes on each side until deeply golden brown and the smell of browning meat fills the kitchen. Remove and set aside to rest, then shred or dice into bite-sized chunks.

Sweat the vegetables. In the same pan (don't wipe it clean!), reduce the heat to medium-low and add the butter. Once melted, tip in the onion, carrot, and celery. Cook for 8-10 minutes, stirring occasionally, until the onions are translucent and the carrots have softened — the kitchen will smell sweet and savoury.

Build the roux and gravy. Add the minced garlic and dried thyme, stirring for 30 seconds until fragrant. Sprinkle over the flour and stir constantly for 2 minutes until the mixture turns a pale biscuit colour and smells toasty. Slowly pour in the hot chicken stock, whisking vigorously to avoid lumps. The sauce will thicken and turn glossy.

Finish the filling. Stir in the double cream and bring the sauce to a gentle simmer — it should be thick enough to coat the back of a spoon. Add the shredded chicken and frozen peas. Taste and adjust seasoning with salt and pepper. The filling should be rich and creamy with a hint of thyme. Remove from the heat and allow to cool slightly.

Assemble the pie. Preheat your oven to 200°C (180°C fan). Roll out two-thirds of the chilled pastry on a lightly floured surface to a circle about 30cm in diameter and 5mm thick. Line a 23cm pie dish, leaving an overhang. Spoon in the chicken filling. Roll out the remaining pastry for the lid, place it on top, and crimp the edges to seal. Cut a few slits in the top for steam to escape.

Bake to golden perfection. Brush the top with beaten egg and place the pie on a baking tray (to catch any drips). Bake for 40-45 minutes until the pastry is a deep, burnished gold and you can see the filling bubbling through the slits. The sound of a bubbling pie is the sound of success. Let it rest on a wire rack for 15 minutes before serving — this sets the filling so it doesn't flood your plate.