A couple years ago I bought a seed packet on line for "various lettuces." Corn salad popped up all over my garden. I had no idea what it was, but was eventually able to find it online and found that it is edible. I harvested it along with the other greens, and none of my tortoises ate it.
I'm going to grow some and try it. Hopefully Geronimo will like it. So far he is a fan of dandelions leaves, romaine lettuce ( which I want to replace the romaine with lambs lettuce ) and cactus. Those are his favorites. So far I'm lucky in that he will eat about anything I give him, but if one of those three are in the mix they get eaten up in a hurry.