method
1. Peel and finely chop onion. Peel and chop or grate potatoes. Melt butter in a saucepan, add onions and potatoes and turn till coated in butter. Cover and cook gently for 10 minutes.
2. Wash, dry and chop watercress leaves and stalks, reserving some whole leaves. Add to pan with stock and bring to boil. Cover and simmer for 15 minutes.
3. Rub soup through sieve or puree in blender. Return to pan, add reserved watercress leaves and reheat. Taste and adjust seasoning. Serve immediately with side dish of whipped cream, or cool, stir in cream and chill well before serving.
