The scientific name, Manjula Desai tells me, is
This belongs to the Capparaceae (Caper) family.
It's called "bhoo chakra gadde" in Kannada, (it would be better translated as "bhoo shakkar gadde"...earth-sugar tuber". I am not able to find the name in Tamil.
Here's a video of a slice being smeared with lemon, dipped in sugar, and served:
Here's another video, where the skillfully-carved slice is served without lemon or sugar:
Someone says it is from Haridwar.
is a scientific study of the DnA of this root.
By Subhashish Panigrahi - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=56546985
Could someone help with more information?