Brown the chicken: Melt 30g of the butter in a large, heavy-based saucepan over a medium-high heat. Add the chicken chunks in a single layer (work in batches if needed). Cook for 5-6 minutes, stirring occasionally, until the chicken is golden brown on all sides and no longer pink in the centre. You'll hear a satisfying sizzle as the meat sears. Remove the chicken with a slotted spoon and set aside on a plate.

Soften the vegetables: Reduce the heat to medium and add the remaining 120g of butter to the same pan. Once it has melted and is gently bubbling, add the diced onion, carrots, and celery. Cook for 8-10 minutes, stirring regularly, until the onions are translucent and the carrots have softened. The kitchen will begin to smell sweet and earthy, and the vegetables should look glossy and tender.

Make the roux: Sprinkle the flour over the softened vegetables and stir constantly for 2 minutes. The mixture will look like a thick, sandy paste. Keep stirring until it turns a pale, biscuit colour and smells slightly nutty. This step is crucial for a smooth, lump-free gravy.

Build the gravy: Slowly pour in the chicken stock, a little at a time, whisking vigorously after each addition. Once you have a smooth sauce, pour in the milk, continuing to whisk. Bring the mixture to a gentle simmer. You will see it visibly thicken and become glossy. Let it bubble for 2-3 minutes, then stir in the frozen peas, thyme, salt, and pepper. The sauce should be thick enough to coat the back of a spoon.

Combine and cool: Return the browned chicken to the pan, along with any juices that have collected on the plate. Stir everything together, then taste and adjust the seasoning. Pour this glorious, steaming filling into a 23cm x 23cm (or similar 2-litre capacity) baking dish. Set it aside to cool for at least 15 minutes. A warm filling will make the pastry soggy, so patience is key here.

Top with pastry: Preheat your oven to 200°C (180°C fan). Unroll your pastry sheet and lay it over the cooled filling. Using a sharp knife, trim the excess pastry, leaving about a 2cm overhang. Crimp the edges with a fork or your fingers to seal. Cut a few small slits in the centre of the pastry to allow steam to escape during baking — this prevents a soggy bottom.

Egg wash and bake: Brush the entire pastry surface with the beaten egg. This will give you that deep, burnished gold finish. Place the dish on a baking tray to catch any drips. Bake for 40-45 minutes, until the pastry is puffed, deeply golden, and the filling is bubbling up through the slits. You should be able to hear a gentle, contented bubbling sound. Let it rest on the counter for 10 minutes before serving. This allows the filling to set slightly so it doesn't flood the plate.