Prepare the leeks by removing any tough outer leaves. Cut leek in half lengthwise, and then slice each half crosswise into half-moons, about ½ inch wide. Use only the white and light green parts, discarding the dark green. Place the cut leeks in a large bowl of water, and agitate to remove any dirt. Lift the leeks out of the water, leaving sediment in the bowl. Repeat if needed.
In a large heavy saucepan over medium-high heat, melt the butter and add the leeks. Cook the leeks with salt and pepper to taste over moderately low heat, covered, stirring occasionally, for 8 to 10 minutes, until they are softened but not browned.
Add the water, stock and potatoes. Bring to a boil then lower the flame to medium-low and simmer for 20 minutes, or until the potatoes are tender.
Puree the soup with an immersion blender or in batches in a blender. Always be cautious when blending hot liquids. Add cream and vinegar or Tabasco to taste and adjust for seasoning with salt and pepper.
Evenly divide the soup among 6 soup bowls and garnish with the herbs.
*May be made vegetarian by using vegetable stock.
