Heat 1 Tbsp. oil in Dutch oven over medium heat, add onion and saute until softened, about 5 to 6 min. In a small bowl, mix together curry, fish sauce and water to make a paste. Stir paste into onions and cook for 1 min. or until fragrant. Add pumpkin puree and stir until combined. Add stock and stir to combine. Bring to a boil, add Kaffir lime leaves, then reduce heat and simmer for 20 min. Add coconut milk and simmer for 10 more min. Remove Kaffir lime leaves before serving.
Top with cilantro and a dash of fish sauce, if desired. Make the soup a little heartier by serving it over cooked white or brown rice.