In a large frying pan, melt 20g of butter over medium-high heat. Add the smoked bacon lardons and sauté for about 5 minutes until they are lightly caramelized and golden.

Once the bacon is done, pour in the thick rounds of leeks and sauté for an additional 5 minutes. Stir occasionally until the leeks soften and become slightly translucent. Pour in the dry white wine and allow it to bubble and reduce completely.

In a separate saucepan, melt the remaining 50g of butter over medium-low heat. Gradually stir in the plain flour to create a roux, cooking it for about 1 minute until lightly golden.

Slowly whisk in 250ml of whole milk and 250ml of chicken stock to the roux, stirring until the mixture thickens into a creamy sauce.

Remove the saucepan from heat and stir in the double cream and wholegrain mustard. Fold in the cooled cooked roast chicken pieces, sautéed leeks, salt, and pepper. Mix in the frozen peas and chopped parsley.

Spoon the filling into a pie dish measuring 28x18cm and 5cm deep, spreading it evenly. Roll out the puff pastry to cover the pie, ensuring an overhang on the edges.

Brush the rim of the dish with the beaten egg. Lay the pastry over the pie, brushing the entire surface with the egg wash and cutting a small hole for steam to escape.

Place the assembled pie in the refrigerator to chill for 30 minutes. Preheat the oven to 200°C/Fan 180°C/Gas 6.

Bake the pie in the preheated oven for around 40 minutes until the pastry is golden brown and crisp. Allow it to cool for about 10 minutes before slicing.