Heat the oil in a large saucepan over medium heat. Add the sliced leeks and cook gently for about 5 minutes until softened.

Add the cubed chicken thighs and cook until sealed and lightly browned on all sides.

Stir in the butter and let it melt. Add the flour and cook for 1 minute, stirring constantly to form a roux.

Gradually pour in the chicken stock, stirring continuously until the sauce thickens and becomes smooth.

Stir in the double cream and chopped tarragon, if using. Season with salt and black pepper to taste.

Spoon the chicken and leek mixture into a 1.2-liter (2-pint) pie dish and spread evenly.

Preheat the oven to 200°C (fan 180°C / gas 6).

Lay the puff pastry over the pie dish, trim the edges, and crimp to seal. Cut a small slit in the center to allow steam to escape.

Brush the pastry with beaten egg.

Bake for 25–30 minutes, until the pastry is puffed, crisp, and golden brown.