What is this plant, and is it edible?
This is a leaf vegetable called Cichorium, Endive in English. There are several closely related species, Cichorium endivia (also called endive), Cichorium pumilum (also called wild endive), and Cichorium intybus (also called common chicory). I think it looks most like common chicory, but if it comes from your seeds, Cichorium endivia is more likely.

Flower from Cichorium intybus 


Flower and leaves of Ch. pumilium

https://en.m.wikipedia.org/wiki/File:Cichorium_pumilum.jpg
(source pictures:wikipedia)