"This recipe is a velvety smooth and very flavorul version of potato and leek soup, without the bacon or salt pork that many other versions contain. A very elegant addition to impress your guests with."
Melt butter in a Dutch oven or a large, heavy pot over medium heat. Add leeks and onion; saute' until tender but not browned, about 12-15 minutes. Add potatoes, water, and 2 cups of the chicken broth; bring this mixture to a boil. Reduce the heat down to medium, cover and cook until the potatoes are tender, about 20 minutes. Puree' soup in small batches, using your blender or food processor or food mill, puree the soup mixture until smooth and no lumps remain. Return the pureed soup mixture to the pot and mix in the heavy whipping cream and 2 cups of chicken broth. Bring to a simmer. Season to taste with salt and pepper. Ladle soup into bowls. Sprinkle with chives and watercress.
Note:You can make this 1 day ahead. Cool the soup slightly. Refrigerate, uncovered until completely cold, then cover and keep refrigerated until you are ready to use it, the next day. Bring to simmer before serving, thinning out the soup with additional chicken broth, if desired).