Serves 4

2 bunches of watercress

25 g (1 oz) English butter

1 small onion, skinned and finely chopped

350 g (12 oz) potatoes, peeled and diced

450 ml (3/4 pint) chicken stock

300 ml (1/2 pint) milk

salt and freshly ground pepper

142 g (5 oz) natural yogurt or 150 ml (5 fl oz) fresh single cream

1. Wash the watercress and reserve a few sprigs to garnish. Cut away any coarse stalks. Chop the leaves and remaining stalks.

2. Melt the butter in a large saucepan, and gently fry the onion and watercress for about 1 5 minutes until soft. Add the potatoes, stock, milk and seasoning. Bring to the boil and simmer gently for 30 minutes, stirring occasionally.

3. Rub the soup through a sieve or puree in a blender. Reheat gently with 60 ml (4 tbsp) of yogurt or fresh cream but do not boil.

4. Pour into a soup tureen or individual bowls, serve garnished with chopped watercress sprigs. Serve the remaining yogurt or fresh cream separately.