Lynn Alley, the author of Lost Arts, and I were talking about this soup on a segement of The Radio Kitchen. It wasn't in her book, but I came up with my own recipe.
Melt the butter in a saucepan over medum low heat. Add the onions and leeks and sweat them until they are about half cooked. Do not brown.
Add the stock and the potatos. Bring to a boil. Simmer until vegetable are tender. Add the chopped cilantro. Remove from heat.
Puree the soup by passing it through a food mill, or using a imersion blender, or by CAREFULLY (* see note) using a blender.
Bring the soup back to a simmer. Add the cream, if you desire. You can add a bit of stock to regulate the consistancy. Season to taste.
* If you are going to use a blender to puree the soup, you need to take extreme care. Start by laddling the soup into your blender to about 1/3 filled. Be sure the top is on the blender firmly, even hold the top in place with a towel in your hand.