Bring a large pan of salted water to the boil, stir in the pasta and return to the boil. Turn down to simmer for about 8 minutes. Drain and set aside.
Whilst the pasta is cooking gently heat the oil in a large sized sauce pan on a medium high heat and add the diced chicken breasts. Cook, stirring to prevent any burning until no longer white, remove from the pan and set aside.
Add the leaks to the pan and gently sauté for about 6 minutes until softened then add the sliced mushrooms and garlic and sauté together for a further 2 minutes.
Add the cream cheese, flour and milk to the pan and stir well together with the vegetables until there are no lumps, then add the stock and stir to combine to a sauce consistency.
Season to taste with the salt and pepper.
Add the chicken back to the pan and then drained pasta, stir through and allow to simmer for a couple of minutes.
Transfer the contents of the saucepan to a casserole dish and top with the mozzarella cheese.
Bake for about 20 minutes until the cheese has melted on top and is golden coloured.
Serve with extra vegetables or a mixed leaf green salad.