I used to work with an old Polish guy, that was also a gardener, and grew a huge amount of parsley root - a.k.a. Hamburg parsley. He was born here, but his parents were from Poland, and that was one of the things they couldn't find anywhere here, and had to grow, if they wanted it, and he carried on the tradition, since he loved the flavor of it. I used to see it only rarely, years ago at the Italian Market, and grew it a few times, but I use the leaves more than the roots! I have found regular flatleaf, and gigante flatleaf get huge roots, as well - too large for hydroponics, or those windowsill boxes on my deck.
Another thing - not a root vegetable, but almost like one - is kohlrabi, which is milder, and is great in soups, in place of turnips. And a great turnip, which I have never seen in stores, except very seldom in Asian markets, is Shogoin turnips - a very mild turnip, which I discovered purely by accident, when I got a seed packet of "Asian Greens", and nobody knew what it was! (One of the problems with buying "mixes"
) This was in the 90s, and the Internet was in its infancy, and it was several years before I saw these things in a catalog, and a couple of years later the place I got the Asian mix from, also had them. Anyway, they grow about 4" in diameter, and almost entirely above ground, even though it is a turnip! The shogoin is mild, as well as the greens, compared to regular turnips; the greens are a little prickly, though that cooks out.