Melt the butter in a saucepan. Add the onions leeks and celery with some salt and pepper.
Cover with a lid and cook over a gentle heat until soft but not coloured. Add the rest of the vegetables and the thyme and just cover with stock. Put the lid back on and simmer until the vegetables are completely soft. Add the cream. Liquidise and check the seasoning. Add more stock if the soup is too thick.
Add the chopped parsley and reheat before serving.