Heat a tablespoon of oil in a large frying pan over medium heat. Add the chopped leeks and cook for about 5 minutes until they are softened and translucent.

Include the cubed chicken thighs, browning them on all sides for another 5-7 minutes until they are lightly golden.

In the same pan, melt 25g of butter over medium heat. Stir in 30g of plain flour for 1 minute to create a roux, mixing until it is smooth.

Gradually whisk in 500ml of chicken stock, cooking until the mixture thickens, about 3-4 minutes.

Incorporate 150ml of double cream and a teaspoon of tarragon, seasoning with salt and pepper to taste.

Pour the creamy chicken and leek filling into a pie dish, spreading it evenly.

Roll out the ready-rolled puff pastry over the dish, trimming any excess. Crimp the edges securely to seal in the flavors, and make a small steam vent in the center.

Preheat your oven to 200°C (fan 180°C) or gas mark 6. Brush the pastry with a beaten egg and place the pie on a baking tray.

Bake for 25-30 minutes or until the pastry is puffed and golden brown, and the filling is bubbling slightly at the edges.