Peel, wash and chop the potatoes into small pieces. Wash the celery and cut into pieces. Peel and dice the onions. Cut half of the watercress. Lightly braise the onions, celery and potatoes in oil for about 1-2 minutes. Pour in the stock, cover and simmer for about 20 minutes. Once the potatoes are soft, add the watercress and purée the soup. Season the soup to taste with salt, pepper and a little lemon juice. Add 3 tbsp cream and half of the prawns. Pour the soup into bowls. Add the remaining cream to the soup in streaks. Garnish the soup with the remaining prawns and watercress. If the soup appears too thick, it may be diluted with stock or a little cream.