In a large frying pan, heat 2 tablespoons of vegetable oil over medium heat. Add the sliced leeks and cook for 5 to 7 minutes, stirring occasionally, until they're soft and translucent.

Add the cubed chicken thighs to the pan with the softened leeks. Increase the heat slightly, and fry the chicken for about 5 minutes until it has sealed and is no longer pink.

Sprinkle 3 tablespoons of plain flour over the chicken and leeks, stirring well to form a roux. Gradually whisk in 500ml of chicken stock, allowing it to thicken smoothly.

Once the sauce has thickened, lower the heat and stir in 200ml of double cream and 1 tablespoon of fresh tarragon. Season with salt and pepper to taste.

Transfer the chicken and leek filling into a large ovenproof dish, spreading it out evenly. Allow it to cool for about 10 minutes.

Roll out your 320g of puff pastry over the top of the filling, sealing the edges with your fingers and cutting a small steam vent in the center.

Preheat your oven to 200°C (400°F). Brush the pastry with a beaten egg and place the pie in the oven. Bake for 25 to 35 minutes until the pastry is puffed and golden brown.