Preheat the oven to 200°C/180°C fan. Peel and chop the potatoes into bite-size pieces. Add them to a pan of boiled water with a generous pinch of salt. Cook the potatoes over a medium heat for 15 min or until soft.

Meanwhile, wash and finely slice the leeks. Chop the parsley. Grate the Parmesan. 

Heat a pan over a low heat with a drizzle of oil. Once hot, add the chicken and chopped leeks with a pinch of salt and cook for 7 min until the leeks are soft and the chicken is browned. Add the water and stock cube and simmer, covered, for 8 min. 

After 8 min, using two forks, pull the chicken until it's fully shredded. Add the cream cheese, parsley, peas and mustard. Give everything a good mix up. Season with salt and pepper and set aside. 

Once soft, drain the potatoes and return them to the pan with a splash of milk. Mash until smooth. Add the Parmesan and season with salt.

Transfer the chicken sauce to an oven proof dish. Carefully top with the mashed potato. Run a fork over the mash to create a ripple effect. Bake for 15 min until golden brown on top.