Traditional recipe https://www.meilleurduchef.com/en/recipe/endive-ham-gratin.h...

But first the crop needs a whole season in the sun. You grow normal green endive, pull the whole plant, chop the head, then you’re growing the second flush in the dark from the root mass. Effectively using all the stored energy from the root for one final round of leaf production, but no light prevents photosynthesis and you get Belgian endive.

Just like with rhubarb.

Similar. Rhubarb is perennial. You're forcing the first flush of growth for the year but you can't force it all season. Later harvests will look different. Still edible, but you're never pulling the entire plant out of the ground with rhubarb like you have to do for Belgian endive.