Heat the olive oil in a pan and gently fry the leeks until just tender. Remove the leeks from the pan, turn up the heat slightly and add the chicken pieces. Quickly seal the chicken on all sides before adding the mushrooms and cooking for a further 2-3 minutes. Add the white wine and thyme leaves to the pan and cover tightly, simmering for 15 -18 minutes, until the chicken is cooked.
Using a slotted spoon, remove the chicken and mushrooms from the pan. Bring the remaining liquid to the boil and allow to reduce slightly. Stir in the creme fraiche and season with salt and black pepper. Cut the chicken into 2cm x 2cm dice and return to pan with the mushrooms and leeks. Stir thoroughly, then allow to cool.
Preheat the oven to 200°C/180°C fan/400°F/gas6. Unroll the pastry sheet and cut out 3x12cm discs and 3x10cm discs.
Lay the larger 3 discs on a baking sheet lined with baking paper, then divide the cooled chicken mixture between them, spreading it evenly over the pastry but leaving a roughly 2cm border around the circumference. Brush the edges with beaten egg, place the smaller pastry discs on top of the filling and bring up the sides of the larger pastry discs, pinching to seal.
Brush the pies all over with the remaining beaten egg and make a small hole in the top of each lid to allow steam to escape. Transfer to the preheated oven and bake for 18-20 minutes, until golden brown.
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