- Preheat the oven to 190°C.
- Melt half the butter in a large, deep-sided pan over a medium heat. Add the leeks and cook for 5 minutes or until softened. Add the bacon and continue to cook for 4 minutes or until the bacon begins to turn golden. Add ¼ cup of cider and continue to cook for 3 minutes.
- Meanwhile, melt the remaining butter in a medium saucepan over a medium heat. Once the butter has melted, add the flour and stir continuously for around 1 minute. Remove the pan from the heat. Gradually add the milk in small amounts to the pan. Stir well after each addition to incorporate the liquid into the flour mixture. Add the remaining cider and then stir to combine.
- Return the pan to the heat and bring to the boil, stirring continuously. Once the mixture reaches the boil, reduce the heat to low, and then stir for a further 4 – 5 minutes until a thick sauce forms. Remove the pan from the heat, and then stir through the leeks, bacon, mustard, and cooked chicken. Season with salt and pepper.
- Return the pan to the heat and stir the mixture for 3 – 4 minutes. Transfer into an ovenproof pie dish and set aside.
- To make the pie topping, place the flour, breadcrumbs and butter into a large bowl. Use your fingertips to combine the ingredients, and then stir through the parmesan and parsley. Season with salt and pepper, and then spread across the top of the pie filling.
- Carefully transfer the pie into the oven to cook for 30 minutes or until golden brown.