Chicken and Leek Pie

Allow 20 minutes for this preparation, with oven heating alongside it. Set the oven to 200°C, or 180°C fan, gas 6; leave it on ready for baking. Use a deep ovenproof pie dish of at least 3 litres, approximately 30 x 22cm inside at the top and 6cm deep, with an outside rim no larger than 32 x 24cm, plus a baking tray. Check its capacity before cooking if unsure. Have a 30cm wide, deep frying pan and two shallow heatproof containers ready. Trim, split lengthways, wash between the layers and slice the leek into 5mm half-moons; aim for about 400g trimmed from the two leeks. Slice the onion, chop the garlic, strip the thyme leaves and measure the stock, crème fraîche, mustard and 30g flour. Cut the chicken into 3cm pieces, pat dry and season lightly with salt and black pepper. Wash hands, board and utensils after handling raw chicken. Keep the pastry chilled until assembly.

Heat the empty frying pan over a medium-high heat for 2 minutes. Divide 1 tbsp of the oil between three batches of chicken, roughly 330g per batch. Brown each batch for 5 minutes, turning to colour several sides, then transfer to a plate; allow 15 minutes for all three batches. Keep the pieces in a single layer with space between them so they sear rather than steam. They should have golden patches but need not be cooked through yet; they will finish in the sauce. Keep the juices on the plate for the filling.

Reduce the hob to medium-low. Add the remaining 1 tbsp oil, the leek and onion, and a small pinch of salt. Cook for 10 minutes, stirring and scraping up the chicken juices, until the vegetables are soft and the onion is translucent, without dark browning. Stir in the garlic and cook for 1 minute until fragrant.

Scatter over the 30g plain flour and stir for 1 minute over a medium-low heat so no dry patches remain. Over the next 5 minutes, gradually stir in the 300ml chicken stock, scraping the base and stirring out lumps; bring to a gentle simmer and cook until the sauce thickens. Stir in the crème fraîche, wholegrain mustard and stripped thyme leaves. The sauce should coat a spoon rather than look like broth.

Return the chicken and all its plate juices to the sauce. Allow 12 minutes over a medium heat to return to a gentle simmer and cook, stirring regularly and reducing the heat once bubbling so it does not catch. The chicken must be cooked through before cooling: use a clean food thermometer in several of the largest pieces, including those in the centre of the pan, and check that they reach 75°C for at least 30 seconds. Include this check in the cooking stage; cook longer if needed. The sauce should cling thickly to the chicken, without a pool of watery liquid. If loose, continue simmering uncovered until it coats the spoon thickly; the cue takes priority over the estimated time. Taste only once the chicken is cooked, then adjust salt and black pepper.

Divide the cooked filling between the two shallow heatproof containers, spreading it out rather than leaving it in the deep pan. Allow about 25 minutes to cool, stirring occasionally, until warm rather than hot and no longer giving off visible steam. Leave the pastry in the fridge and the oven on during this stage. Do not cover hot filling with pastry; allow longer cooling if it is still steaming. This method goes straight from this short cooling stage to assembly and baking, not into a make-ahead refrigerator or freezer branch.

Allow 10 minutes for assembly. Spoon the cooled filling into the 3-litre pie dish, leaving at least 2cm clear below the rim; do not use the former 1.5-litre dish. Lightly dust the worktop with plain flour. Roll the 500g puff pastry block to approximately 36 x 28cm and 4mm thick, keeping the layers even. It must extend beyond the dish's outside rim on every side. Brush the rim lightly with beaten egg, lift the pastry over the filling and press it onto the rim without stretching. Trim to a small overhang and crimp to seal. There is no pastry base. Cut two slits through the lid near the centre to release steam, then brush the top with beaten egg, avoiding the exposed cut edges so they can rise. Discard unused raw egg glaze. Put the dish on the baking tray.

Bake on the middle shelf for about 40 minutes at 200°C, or 180°C fan, gas 6, until the lid is well risen, deep golden and crisp, with sauce bubbling through the vents. Check near the end of this stage with a clean food thermometer through a vent: the centre of the filling and several large chicken pieces there should reach 75°C for at least 30 seconds. Edge bubbles alone do not prove the centre is hot. Continue baking if needed to meet both pastry and centre-temperature cues, covering the top loosely with foil if it is browning too quickly. The thermometer check is included in the baking allowance; the safe endpoint takes priority over the clock.

Rest the pie for 5 minutes before serving. Divide the lid into six portions and spoon the creamy filling underneath each portion; this is a lid-only pie, not a firm slice with a pastry base. Serve with peas or steamed greens. The filling will still be hot.