Heat a splash of oil in a large pan over medium heat. Add the sliced leeks and sauté them for about 5 minutes until they soften and become translucent.

Stir in the cubed chicken thighs, cooking for an additional 8-10 minutes until they are browned and fully cooked.

In the same pan, melt 2 tablespoons of butter over low heat. Once melted, sprinkle in plain flour and stir continuously for 2 minutes to create a roux.

Gradually pour in chicken stock, whisking until the sauce thickens and becomes smooth—this will take about 5 minutes.

Stir in double cream and chopped tarragon, seasoning with salt and pepper to taste.

Transfer the chicken and leek filling into a pie dish, ensuring an even distribution for optimal flavor.

Preheat your oven to 200°C (fan 180°C/gas 6) while you roll out the ready-rolled puff pastry.

Drape the pastry over the filled pie dish, trimming any excess dough from the edges, and crimp the edges together.

Brush the pastry with a beaten egg, then bake in the preheated oven for 25-30 minutes, or until the pastry puffs up and turns a rich, golden brown.