Preheat oven to 180° C (160°C for fan assisted ovens) Gas Mark 5.
Place the chicken thighs in a roasting tin, drizzle with a little oil and scatter with a few sprigs of thyme. Roast for 20 -25 minutes, remove from oven and allow to cool. Once cooled remove skin and bone from meat and break meat into medium sized pieces.
Meanwhile melt the butter in a largish frying pan and gently sauté the leeks until just ‘al dente’. Add the juice and grated rind of the lemon, stir in the flour and then gradually stir in the stock to give a good sauce. Bring to the boil to thicken and then simmer for a further 3-4 minutes. Remove from the heat, add the chicken and crème fraiche, season to taste and allow to cool. Increase the oven temp. to 220°C (200°C) Gas Mark 7
Once cooled put leek and chicken filling into an oblong pie or other ovenproof dish and brush the top rim with beaten egg and top with the light puff pastry sheet.
Press well down all round the top edge, trim any excess about 2cm down sides and press pastry firmly against side of dish. Make a slit or hole in the top and brush with beaten egg. Bake for approximately 20 minutes until pastry golden and risen.