Heat the butter in a large saucepan over medium heat. Add the onion, celery and garlic. Cook until the onion is translucent, about 10 minutes. Add the pumpkin puree, vegetable broth, water, curry, brown sugar, cumin, pepper, salt and nutmeg (everything else but the thyme). Reduce the heat to moderately low and simmer for 30 minutes, stirring occasionally. Transfer the vegetable soup to a blender (working in batches) and puree until smooth. Add the thyme and taste for additional seasoning. Ladle the soup into bowls and serve.