For the watercress soup, chop the onion and garlic into cubes and sauté in butter. Peel the potato and grate it on a fine grater and add to the onions. Pour in the soup and simmer on low heat with the lid closed for 20 minutes.
Sort out the watercress, add the leaves to the saucepan and cook for about five minutes. Finely chop with a hand blender to a puree, then blend in the crème fraîche.
For the croutons, cut the bread into small cubes and fry in hot olive oil until crisp.
Season the watercress soup with salt, pepper and Worcestershire sauce and serve with wholemeal bread croutons.