Trim the cauliflower, separate into pieces, and boil in 4 cups water. Add chopped potatoes and cook until soft. Trim scallions, setting aside green tops. Chop white onion parts and add to soup pot. Cook about 20 minutes, or until everything is soft. Transfer to blender and puree, adding half-and-half. Chill until very cold, then season with salt and pepper, and serve with chopped chives floating on top.