Begin by heating 1 tablespoon 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 almost translucent. Next, add the skinless, boneless chicken thighs, browning them on all sides for around 8-10 minutes until they are lightly golden and cooked through.

In the same pan, reduce the heat to low and melt 2 tablespoons of butter. Stir in 2 tablespoons of plain flour, cooking for about 1 minute to form a roux. Gradually whisk in 400ml of chicken stock, ensuring there are no lumps, and continue stirring until the mixture thickens, which should take about 3-4 minutes.

Once the sauce is thickened, gently stir in 200ml of double cream and the optional fresh tarragon. Season your filling with salt and freshly ground black pepper to taste, allowing the flavors to meld together. Simmer it for another 2-3 minutes, making sure the mixture is nice and creamy.

Transfer the flavorful chicken and leek mixture into a pie dish, spreading it out evenly to ensure every slice is packed with delicious filling. Allow the filling to cool slightly while you prepare the pastry.

Preheat your oven to 200°C (fan 180°C/gas 6). Roll out your ready-rolled puff pastry on a lightly floured surface. Cover the pie filling completely with the pastry, trimming any excess, and crimp the edges together to seal. Cut a small steam vent in the center for steam to escape during baking.

Brush the surface of the pastry with a beaten egg for that golden finish. Place your pie in the preheated oven and bake for 25-30 minutes, or until the pastry is puffed and beautifully golden brown.