PLACE squash and carrots in large pot. Add broth and water; stir. Bring to boil over medium-high heat; cover. Reduce heat to medium-low; simmer 10 minutes or until vegetables are tender. Cool slightly.

PUREÉ in blender in small batches; covered. Return to pot; keep warm on low heat.

STIR IN pepper and nutmeg. Remove from heat. Add a few tablespoonfuls of hot soup to sour cream in a small bowl; mix well. Stir into hot soup. Top with additional sour cream if desired.