To make soup: Heat the olive oil in a large saucepan over medium heat until hot. Add the onion, celery, carrot and cinnamon stick; sauté until soft but not brown, about 10 minutes. Season with salt and pepper. Add the broth and the coriander; bring to a boil. Simmer for several minutes. Stir in reserved squash until smooth; simmer gently to let the flavors meld, about 10 minutes. Discard the cinnamon stick.