Preheat the oven to 200C or 180C Fan or 400F or Gas 6
Fill a pan with water filled with the leek trimmings, a few thyme sprigs, and a chicken stock pot to a boil and reduce to a simmer. Add the chicken breasts, cover the pan and poach for 12-15 minutes until the juices run clear or a meat thermometer reaches 75 °c.
Remove the chicken from the stock and set aside to cool slightly. Strain the stock and reserve the liquid.
Heat a large pan over a medium heat and melt half of the butter.
Add the leeks and a pinch of salt. Cook for 4-5 minutes until tender.
Stir in the flour and cook for another minute
Slowly stir in the reserved stock, stirring to prevent lumps. Once finished, add the cream and mustard. Simmer for a minute or two while preparing the chicken.
Carefully dice the cooked chicken thighs into 2.5 cm chunks and add to the pan.
Pour the chicken mixture into a pie dish
Melt the remaining butter and lightly brush the sheets of filo pastry. Once buttered, gently crumple the sheets and top the pie.
Place it in the centre of the preheated oven for 20-25 minutes or until crisp and golden.