1. In a 4-quart Dutch oven, heat oil over medium heat. Add onion; cook for 3 minutes, stirring occasionally. Add curry paste and garlic; cook for 3 to 5 minutes or until onion is tender, stirring occasionally. Stir in sweet potato, chicken broth, coconut milk, lemongrass, and, if desired, ginger.
2. Bring mixture to boiling; reduce heat. Cover and simmer for 10 to 15 minutes or until sweet potato is tender. Cool slightly. Discard lemongrass.
3. Transfer half of the potato mixture (about 4 cups) in small batches to a blender or food processor. Cover and blend or process until smooth. Return pureed mixture to the remaining mixture in Dutch oven. Stir in white pepper and salt. Heat through. Makes 10 servings.
4. Make-Ahead Tip: Prepare and blend chowder as directed. Transfer to an airtight container and chill for up to 24 hours. To reheat, transfer to a Dutch oven. Cover and cook until heated through.