Heat a splash of oil in a large frying pan over medium heat. Add sliced leeks and sauté for about 5 minutes until soft and fragrant.

Add cubed chicken thighs to the pan and cook for 7-10 minutes until golden brown and no longer pink.

In the same pan, melt 30 grams of butter over low heat and stir in 30 grams of plain flour to create a roux.

Cook the roux for around 1-2 minutes, stirring constantly, then gradually whisk in 500ml of chicken stock.

Continue to stir until the sauce thickens, about 3-5 minutes, then add 150ml of double cream and fresh tarragon, seasoning with salt and pepper.

Spoon the chicken and leek mixture into a 1.2-liter pie dish, spreading it evenly. Let the filling cool slightly.

Preheat your oven to 200°C (fan 180°C or gas mark 6). Roll out the ready-rolled puff pastry over the pie dish.

Trim excess pastry and crimp the edges to seal in the filling. Cut a small vent in the center.

Brush the top of the pastry with beaten egg before baking. Bake for 25-30 minutes or until the pastry is puffed and golden brown.