Place a good sized, thick bottomed sauce pan on a really low heat and add the butter. As the butter starts to melt add all of the diced vegetables and thyme.
Allow to cook away gently, stirring occasionally. Once all the vegetables are soft and beginning to break down, (this can take anything up to 60 minutes), add the bay leaves and cold water (just enough to cover) and turn up the heat slightly.
Bring the soup slowly to the boil, stirring occasionally, and allow to simmer for a further 10-15 minutes.
Take off the heat and carefully remove the thyme and bay leaves. Pour mixture into a food blender and blend till smooth.
Return to the heat and bring back to temperature, you may need to add a little boiling water if it is too thick. Season to taste, garnish with parsley and serve.