- Melt the butter in a large pan. Add the leeks, potatoes and watercress, and stir well.
- Add a good pinch of salt. Then cover the pan and cook the vegetables over a low heat for about 20 minutes, stirring occasionally.
- Add the stock and bring to simmering point. Cover and simmer for a further 10–15 minutes until the vegetables are tender.
- Remove from the heat and allow to cool.
- Liquidise the cooled soup with a food processor or blender, then return it to the pan.
- Stir in the cream, season with salt and pepper to taste and reheat gently.
- Garnish with watercress leaves and serve.
Leave the soup in the fridge overnight to improve the flavours.