Top and tail the green beans and remove the strings. Wash the beans in a colander under cold running water. Put them in a saucepan and cover with freshly boiled, filtered water. Bring to the boil, then lower the heat and simmer, uncovered, for 4-5 minutes of until tender.
Remove from the pan and purée to a smooth texture in a blender or food processor, using some of the cooking liquid to get the right consistency. Allow the mixture to cool.