In a large saucepan, saute onion in butter until tender. Add the broth, potatoes and pumpkin; cook until the potatoes are tender, about 15 minutes. Remove from the heat; cool. Puree half of the mixture at a time in a blender or food processor until smooth; return all to the pan. Add the milk, nutmeg, salt and pepper; heat through. Meanwhile, combine the sour cream and parsley. Spoon soup into bowls; top each with a dollop of sour cream and sprinkle with bacon.