Heat 2 tablespoons of oil in a large frying pan over medium heat. Add the sliced leeks and sauté for about 5 minutes until they are softened and fragrant.

Stir in the cubed chicken thighs, cooking until they are nicely browned on all sides, around 6-8 minutes.

In the same pan, reduce the heat and add 2 tablespoons of butter, melting it gently.

Sprinkle in 2 tablespoons of plain flour, stirring vigorously for about 1 minute to form a roux.

Gradually pour in 400ml of chicken stock while whisking continuously to avoid lumps.

Cook until the sauce thickens, about 3-4 minutes, then stir in 150ml of double cream and 1 tablespoon of fresh tarragon, seasoning with salt and pepper.

Preheat your oven to 200°C (fan 180°C). Spoon the filling into a 1.2-liter pie dish.

Roll out the puff pastry and drape it over the filled dish. Trim the excess, crimp edges, and cut a small vent.

Brush the top with a beaten egg and bake for 25-30 minutes or until golden brown.

Allow the pie to rest for about 5 minutes before slicing.