Directions
1. Pick off 12 neat watercress leaves and dip them into a small soup pan of boiling water for 2 seconds.
2. Refresh under cold water and use them to garnish the finished soup.
3. Melt the butter in a thick-bottomed pan.
4. Add the onions and leek, then cook for a few minutes without browning, keeping the lid on.
5. Add the stock, potatoes, the rest of the watercress, including the stalks, and the bouquet garni. Season.
6. Simmer for about 30 minutes. Remove the bouquet garni and skim off any fat.
7. Pass the soup firmly through a sieve, then pass through a medium conical strainer.
8. Return to a clean pan, re-boil, correct the seasoning and consistency and sieve.
9. Garnish with watercress and serve hot with toasted croutons separately.