Watercress and Lime Soup

Ingredients

 * 50 g (2 oz) butter
 * 250 g (8 oz) potato, diced
 * 250 g (8 oz) leek, chopped
 * 3 bunches watercress, stalks removed
 * 1 liter (1 3/4 pints) chicken stock, salt and pepper to taste, grated rind and juice of 1 lime
 * 4 tablespoons cream watercress sprigs to garnish

Directions

 * 1) Melt the butter in a large pan. Add the potato and leek, and cook gently for about 10 minutes until soft.
 * 2) Meanwhile finely chop the watercress. Add the chicken stock to the pan, bring to the boil and add a pinch of salt. Add the watercress and simmer for 8 minutes.
 * 3) Transfer to a food processor or blender and puree until smooth.
 * 4) Return the soup to the pan. Add the lime rind and juice, check the seasoning and heat through.
 * 5) Transfer to individual soup plates, swirl in the cream and serve garnished with watercress.