You need a food processor or blender to make the dressing. If you have neither, pound the garlic and walnuts in a mortar and add the vinegar, salt, pepper and cayenne. Mix. Slowly add the oil beating well with a fork. Otherwise, start your machine and put in the garlic. When it is minced, put in the walnuts. When they have turned crumb-like, add the vinegar. Slowly add 4 tablespoons of the reserved bean liquid. Blend until you have a paste. You may have to stop the machine and scrape down a few times. Now add the oil in a steady stream and then the salt, pepper and cayenne. Blend until you have a smooth dressing.

Pour the dressing over the beans. Add half the spring onion and half the coriander mix and taste for seasonings.

You may cover and chill the salad until you are ready to eat; it improves if allowed to sit for a few hours. Serve garnished with remaining spring onions and coriander.