Bring the stock to the boil and add in the leeks, cauliflower and potato. Bring back up to the boil, then turn down and pop on the lid, simmering the soup for 20 minutes. Meanwhile, slice the remaining small leek into rounds and sauté on each side until golden-brown.
Decant the soup into a blender or food processor, along with the Dijon mustard and watercress, and blend until smooth. Serve, topped with the sautéed leeks.