How to Make It
In a large saucepan, melt the butter. Add the onion, leek, fennel and a generous pinch each of salt and pepper and cook over moderately high heat, stirring occasionally, until softened and just starting to brown, 9 minutes. Add the carrots, celery root, apple, gingersnaps, curry powder, garlic, ginger and thyme and cook, stirring, until the carrots and celery root soften slightly, 10 minutes. Add the stock and bring to a boil. Simmer over moderate heat, stirring, until the vegetables are very tender, 25 minutes. Discard the thyme sprigs.
Working in batches, puree the soup in a blender with the sour cream and vinegar until smooth. Reheat the soup if necessary and season with salt and pepper. Ladle the soup into bowls, top with toasted pumpkin seeds and chopped mint and cilantro and serve.