Chicken Stock: Put the chicken in a heavy saucepan or pressure cooker (this is what I use). Add the onions, carrots, celery, water, salt and pepper. Bring to the boil then lower the heat to simmer for a good 40 minutes, or if you use a pressure cooker about 20 minutes.
Soup: Slice the mushrooms and put in a saucepan with the butter.
Fry the mushrooms in butter until just soft then stir in a tablespoon of plain flour add a splash of sherry and season to taste. Heat very gently for a few minutes. (Don't worry if it goes a little lumpy at this stage.)
Remove the chicken from the stock and reserve for another recipe. Pour the stock through a sieve into the saucepan with the mushrooms.
Then blend the mixture with a hand-held blender.
Taste for seasoning then add the cream, stirring gently. Reheat the soup until it just comes to simmer. If you don't use the sherry grate a little nutmeg in instead. Garnish with parsley.