Summer Watercress Soup

2 bunches watercress, trimmed
1 tablespoon polyunsaturated vegetable oil
1 small onion, peeled and finely chopped
600 ml/1 pint chicken stock
2 egg yolks
150 ml/1/2 pint plain low-fat yogurt
salt
freshly ground white pepper

Remove 16 leaves from the watercress and reserve for the garnish. Chop the remaining watercress and stems.

Heat the oil in a pan, add the onion and fry gently until soft and golden. Add the watercress and the stock and bring to the boil. Lower the heat, cover and simmer for 20 minutes.

Rub the soup through a sieve, or work to a purée in an electric blender. Put the egg yolks into a bowl and whisk in the yogurt. Stir in 4 tablespoons of the puree and mix well.

Pour the remaining purée into the rinsed-out pan. Add the yogurt mixture, stirring constantly, then heat through gently without boiling. Add salt and pepper to taste.

Pour into 4 individual bowls and garnish with the reserved watercress leaves. This soup may also be served chilled