Add the mixed mushrooms to the pan and stir until mixed in, cooking on a gentle heat for around 5 minutes until they start to soften and reduce in size.
Add the stock, bring to the boil and then turn down the heat and simmer for 10-15 minutes
Add the watercress, beans and parsley and cook for another five minutes
Turn the pan off, leave to cool for a short while before blending with a stick blender
Serve in bowls and stir in the creme fraiche, if using.