Melt 20g of butter in a medium frying pan over medium heat. Add the smoked bacon lardons and cook until caramelized, about 5 minutes. Add chopped leeks and sauté for another 5 minutes until soft. Pour in wine and reduce slightly, then remove from heat to cool.

In a separate saucepan, melt the remaining 30g of butter over low heat. Whisk in 50g of plain flour to create a roux. Gradually add 250ml of milk and 250ml of chicken stock while whisking, stirring for 2-3 minutes until thickened. Remove from heat.

Off heat, fold in 2 tablespoons of double cream and 1.5 tablespoons of mustard into the roux. Incorporate cooled chicken and leeks, seasoning with salt and pepper. Stir in 100g peas and 2 tablespoons parsley.

Preheat oven to 200°C (Fan 180°C/Gas 6). Spoon filling into a pie dish, spread evenly. Roll out pastry to cover the dish, brush edges with beaten egg, and press to seal. Cut a steam hole in the center.

Chill pie in the refrigerator for about 30 minutes. Bake for approximately 40 minutes until golden brown. Let cool for 10 minutes before slicing and serving.