Strange lettuce

A couple days ago, I was visiting a friend and she was showing me the lettuce she is growing. She gave me a bit to taste and it’s quite different. It’s called brown romaine and starts off tasting like ordinary lettuce. But once you get to the stem, it turns quite hot, like horseradish or something. Her family doesn’t like it so she has given me the seeds and a bag with some of the lettuce leaves. I rather suspect this is not the usual taste for this lettuce because I can find no mention of such a strong flavor for any sort of lettuce (aside from bitterness). We are going to plant seeds in pots and see how it turns out.