Radicchio looks like red cabbage, but the leaf is thinner and less waxy.  You may have already had it in salads and assumed it was red cabbage, but your chompers didn’t have to work nearly as hard.  From what I gather, it’s only ever consumed raw, and it’s usually mixed in with other greens in salads.  If you are taking the challenge, you might save this one for a week when you’re not feeling too ambitious since it’s an easy plus-one with lettuce.