In a large skillet, heat a tablespoon of oil over medium heat. Add the sliced leeks and sauté for about 5 minutes until they become soft and translucent.

Next, add cubed chicken thighs to the pan, cooking until they are evenly browned on all sides, which should take about 6-8 minutes.

In the same skillet, melt 2 tablespoons of butter over medium-low heat. Stir in 2 tablespoons of plain flour, cooking for about 1 minute to create a roux.

Gradually pour in 400ml of chicken stock, whisking continuously to prevent lumps. Cook until the mixture thickens, which will take around 3-5 minutes.

Mix in 150ml of double cream along with chopped fresh tarragon, and season with salt and pepper to taste.

Transfer the filling into a 1.2-liter pie dish, rolling out the puff pastry to fit over the dish.

Cut a small vent in the center of the pastry to allow steam to escape.

Preheat your oven to 200°C (fan 180°C/gas 6). Brush the pastry with the beaten egg.

Bake for 25-30 minutes, or until the pastry is puffed and beautifully golden brown.

Let the pie cool for a few minutes before slicing and serve hot.