How to Make It
Melt the butter in a very large, deep skillet. Add the celery roots and cook over moderately low heat, stirring occasionally, until starting to soften, about 10 minutes. Season the celery root with salt and pepper. Add the chopped apples, verjus, star anise pod and sugar. Cover and cook over low heat, stirring occasionally, until the celery root and apples are very tender, about 30 minutes.
Discard the star anise pod. Working in batches, in a food processor, puree the celery root and apples with the raw spinach. Season with salt and pepper and serve hot.