Preheat the oven to 220C
Peel the potatoes and slice into 1 cm rounds.
Add to a pan of salted boiling water and cook for ten to fifteen minutes until soft
Pour into a colander and set aside
Chop the chicken into small chunks. Add into a pan with hot oil and cook for around five minutes until brown. Set aside.
Cut the leeks and celery into small pieces
Add the leeks and celery to a pan with hot oil and cook gently for five minutes
Chop the mushrooms and add them to the pan. Cook for five minutes until brown
Add the water and stock pot to the pan and stir until it dissolves
Pour in the cream and simmer until the liquid reduces by a third.
Put the chicken back into the pan. Season with salt and pepper and put the chicken mixture in an oven proof dish
Grate the cheese
Top the chicken mixture with the potato slices and sprinkle over the cheese and breadcrumbs
Add the dish into the oven and cook until the top is browned