In a soup pot, saute the onions, celery, and salt in the butter for 5 minutes on medium-high heat. Add the potatoes, water, dill and marjoram, cover and bring to a boil. Reduce the heat and simmer until the potatoes are soft, about 10 minutes. In a blender in batches, puree the vegetable mixture with the milk and, if using, the cream cheese. Return the soup to the pot and gently reheat.
Serves 4-6