1. Heat oil in medium saucepan over medium-low heat. Add celery, potato, onion, sugar, sage and salt. Cover and cook until celery and onion are soft, 12-15 minutes.
2. Stir in broth and simmer until potato is soft, 10-15 minutes.
3. Puree with an immersion blender until smooth. Stir in milk and return to a simmer before serving hot.