Heat the oil in a large pan, add the onion and celery and sauté over a medium heat for 5 mins until pale golden. Stir in the potato and stock and bring to the boil. Cover and simmer for 10 mins or until the potato is tender.

Stir in the watercress, cover and cook for a further 5 mins or until the watercress is wilted. Transfer the soup to a food processor and blitz until smooth. Return the soup to the rinsed out pan and add the milk, nutmeg, lemon juice and seasoning to taste. Gently reheat until piping hot and serve with crusty bread.