Melt the butter in a heavy-bottomed saucepan. When it foams, add the potatoes and onions and toss them until well coated. Sprinkle with salt and freshly ground pepper. Cover and sweat on a gentle heat for 10 minutes. Add the stock and milk, bring to the boil and cook until the potatoes and onions are soft. Add the watercress and boil with the lid off for 4–5 minutes until the watercress is cooked. Do not overcook or the soup will lose it’s fresh green colour. Purée the soup in a liquidizer or food processor. Taste and correct seasoning.
Serve in warm bowls garnished with a blob of whipped cream and a watercress leaf.
© 2001 Darina Allen. All rights reserved.