Step 1Heat butter in medium saucepan over medium-high heat. When hot, add onion. Cook, stirring occasionally, until starting to soften, about 6 minutes. Add broth, celery roots, apple and potatoes. Bring to boil. Simmer, covered, until vegetables are very soft, about 20 minutes. Pour through sieve, reserving vegetables and liquid.
Step 2Transfer vegetables and 1 cup liquid to blender, in batches if necessary. Process until pureed. Add remaining liquid and whipping cream, sugar, nutmeg, salt and pepper to taste. Process until as smooth as possible, at least 1 minute. Taste and adjust seasoning. (Can be refrigerated up to 3 days or frozen in airtight plastic container up to 1 month. To serve, gently reheat soup. If too thick, thin with water.) Ladle into soup bowls and serve hot.