Put the chicken in a large pan, pour over the stock and bring to the boil. Reduce the heat and simmer for 15-20 minutes, until cooked through, with no pink showing. Remove the chicken and cut into medium sized chunks. Reserve the stock.
Meanwhile, melt the butter in a large pan and cook the leeks until just softened. Add the lemon juice and zest, followed by the flour, then gradually the stock stirring until the flour has dissolved. Bring to the boil, then add the thyme and simmer for a further 3-4 minutes until thickened. Remove from the heat and add the chicken. Stir in the crème fraîche and season well.
Increase the oven temperature to Gas Mark 7, 220°C, fan 200°C. Put the chicken and leek mixture in an ovenproof dish and brush the rim with a little beaten egg. Cover the dish with a puff pastry sheet, pressing down firmly with your fingers to seal. Trim the excess pastry, leaving a 2cm (1in) overhang.
Make a small hole in the pastry to allow the steam to escape and brush with the remaining beaten egg. Bake in the oven for 20 minutes until puffed and golden.
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