What is it?
I still don't know.
The folks I bought it from at the greenmarket last month didn't know--though if I remember right, they compared the flavor to spinach--and it isn't to be found in any of my cookbooks.
It resembles puntarelle, but the stalks are thinner and the flavor much more delicate.
I sauteed the greens with olive oil and garlic, added a little cheese, and filled an omelet with them. They were a little chewy but delicious.