A simple and delicious soup made from celeriac, a type of celery root cultivated to grow into a larger size. You can use parsley in place of coriander.
I found this in a recipe listing for dishes that can be made with few ingredients and easily. I added coriander and wrote it down from memory.
It's plenty delicious without the coriander (or parsley), but adding it gives it a nice aroma and increases the delicious flavor.
Enjoy Japanese food around the world.