In a hot frypan, sear the chicken, skin side down with a little oil until the chicken pieces are nicely brown,
In the same frypan, saute your onions, leeks, carrots with butter for a few minutes until soft then saute the garlic, mushrooms. Add herbs.
When all the vegetables are soft and chicken is mostly cooked, stir in your flour and let cook a little. Then slowly add your milk. This will thicken up the mix. You may not need all the milk. Let is cool down and rest.
Place chicken mix at the top, then brush the lip of the pie dish with a little egg wash to seal the puff pastry on top. Egg wash the puff pastry at the top for better colour.
Bake in the oven at 180 deg C for about 20 minutes, or until the puff pastry has risen and become golden.