In a 4- or 5-qt. slow cooker, combine all ingredients except sour cream and pepitas. Cook, covered, on low until potatoes are tender, 5-6 hours., Puree soup using an immersion blender. Or, cool slightly and puree soup in batches in a blender; return to slow cooker and heat through. If desired, top servings with sour cream and pepitas. Freeze option: Freeze cooled soup in freezer containers. To use, partially thaw in refrigerator overnight. Heat through in a saucepan, stirring occasionally; add broth or water if necessary.