First make the filling, heat butter in a pan, add in leeks and saute it on medium heat till it is wilted and cooked, it will take around 8 mins.

Toss chicken cubes in flour and add it right into the pan with leeks. Now saute this whole thing together for a good 5 mins till the chicken is done. Meanwhile season with chicken with salt and pepper.

Now add in dried tarragon and mix well.

Now add in milk and mix well. Cook till it thickens.Now set aside to cool down.

Meanwhile preheat your oven to 220 degree C. Line a baking pan with foil or parchment paper and set aside.Now take your puff pastry sheets and cut into 6 equal portions.

Now spoon generous amount of filling on one side of the pastry, cover with another one and use a fork to seal the edges. Now make a cross in the center of the pie and brush with egg wash or milk or cream.Now pop it into the preheated oven and bake for 20 to 25 mins, till the top is golden brown.Remove it from oven and cool down for 5 mins. Now remove this to a plate and serve.