A traditional Hampshire recipe using local watercress.
Melt the butter in a large pan, add the potatoes, leeks, garlic and watercress, season with salt and stir to coat with the butter.
Cover and heat a very low heat for about 15 minutes, stirring occasionally.
Add the stock or water, bring to the boil, reduce the heat and simmer, covered, for around 15 minutes or until the potatoes are soft.
Allow to cool and liquidise, then stir in the cream or yoghurt and reheat gently.
Be careful with single cream and yoghurt as they can curdle if the temperature gets too near boiling.
Add a few good grinds of black pepper, then check and if necessary adjust the seasoning.
Serve plain, or with a sprig of watercress to garnish.