1. Heat oil in large saucepan over medium heat. Add onion, and sauté 5 to 7 minutes, or until translucent. Stir in pumpkin, hot-and-sour paste, cilantro stems, and kaffir lime leaves. Add 3 cups water. Cover, and bring to a boil. Reduce heat to medium-low, and simmer 20 minutes, or until pumpkin is soft.
2. Remove cilantro stems and kaffir lime leaves; purée soup until smooth. Stir in coconut milk. Serve garnished with cilantro sprigs.