In a large pot, melt the butter. Add the leeks, potatoes and broccoli and cook over high heat for 10 minutes, stirring frequently. Add the broth and 3 cups of water, cover and bring to a boil. Simmer over moderate heat until all of the vegetables are tender, about 10 minutes. Working in batches, puree the soup in a blender until smooth. Return the puree to the pot and stir in the milk. Season the soup with salt and pepper. Garnish with rosemary, if desired. Serve hot with butter sourdough.