Gently fry the leeks in the butter until just tender, remove from the pan. Add the chicken fillets and quickly sear on all sides. Add mushrooms and cook for a further 2-3 minutes. Then add the wine and thyme leaves, cover and simmer for 15-18 minutes until the chicken is cooked through, with no pink showing.
Remove the chicken and mushrooms from the pan, bring the juices rapidly to the boil and allow to reduce slightly, then stir in the crème fraîche.
Meanwhile, cut the chicken into medium dice pieces and return to the pan with the mushrooms and leeks, stir and allow to cool.
Preheat the oven to Gas Mark 6, 200ºC, fan 180ºC. Unroll the pastry and cut out 3x15cm (6in) discs and 3x10cm (4in) discs (you will just achieve this from the sheet, don't worry if there are any small gaps around edges of the discs).
Lay the large discs on a lined baking sheet, divide the chicken mix between them, leaving a border. Brush the borders with beaten egg. Put the smaller pastry discs on top, then bring up the sides of the base pastry to just cover the rims. Pinch the edges to seal and press gently onto the lids. Brush with beaten egg. Make a small hole in top of each lid.
Bake the pies for 18-20 minutes until golden brown. Serve hot or cold.
Freezing and defrosting guidelines
Once the dish has cooled completely, transfer it to an airtight, freezer-safe container, seal and freeze for up to 1-3 months. To serve, defrost thoroughly in the fridge overnight before reheating. Bake until dish is thoroughly heated through. Reheat until piping hot.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.