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 lime leaves, then reduce heat and simmer for 20 min. Add coconut milk and simmer for 10 more min. Remove 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.