1. Heat oil in a Dutch oven over medium heat. Add onion; cook, stirring, 5 minutes, or until softened. Stir in next 8 ingredients (through black pep-per), and bring to a boil over high heat. Reduce heat to low, cover, and simmer 20 minutes, until squash is very soft.
2. Puree soup in batches in a blender. Serve hot, garnished with chives.