potatoes, thinly sliced
1Chop watercress leaves and stems, reserving a few leaves for garnish. Peel and thinly slice potatoes.
2In a kettle, melt the butter and add watercress and potatoes; cover the pan and cook gently for 12-15 minutes or until the vegetables are very soft. Stir often.
3Pour in the stock, bring to a boil, season and simmer 10-15 minutes, stirring occasionally.
4Cool the soup slightly; then work it through a sieve or blender. Return to the pan with the cream; bring just to a boil and taste for seasoning (salt & pepper).
5The soup should be the consistency of very thin cream. It will thicken on cooling; if necessary, add a little more stock.
6If you want to serve cold, Cover the soup tightly, let cool, then chill it.
7Optional: Put a dollop of sour cream in each bowl, garnish with watercress leaves.