Preheat the oven to 190°C. Cook the pasta according to the packet instructions, drain and leave to one side while making the creamy sauce.
Spray a frying pan with low-calorie cooking spray and set on a medium heat. Add the chicken, leeks and garlic and fry for 4-5 minutes until the chicken is browned on all sides. Add the mushrooms and fry for a further 4 minutes.
Add the stock pot to the boiling water, along with the Dijon mustard and stir to combine. Pour into the pan and bring to a gentle simmer for 8-10 minutes until the chicken is cooked through and the sauce has reduced. Remove the pan from the heat and stir in the cream cheese until combined. Season with salt and pepper. Stir through the cooked pasta.
In a bowl, combine the breadcrumbs with the parsley and cheese. Pour the chicken and pasta filling into the oven dish and sprinkle over the breadcrumb mixture. Pop into the oven for 20 minutes until the breadcrumbs are crispy and the cheese has melted.