I wasn't sure what to call this because the thing has too damn many names. Depending on where you live, it's a rutabaga or a swede or a neep or a yellow turnip. Please leave comments with other names if you know what I'm talking about but call it something else. This is where Latin species names come in handy, because no matter how many other names it has and might share with other plants, it can unambiguously be referred to as Brassica napobrassica.
The genus Brassica also includes cabbages, cauliflower, broccoli, and Brussels sprouts. If you are not a fan of that stuff and are going 'ick' right now, remember that turnips and rutabagas are the roots of plants, not the leaves and flower heads and stems, and they taste very different.
A rutabaga looks formidable in the produce department -- a big ugly yellow-and-brown streaked thing, like a giant gas planet only lumpier. In my part of the world they're harvested in the fall, and stored with a wax coating so that they're available right through until the next summer. They're cheap, usually sold per kilogram at a price comparable to that of beets or sweet potatoes.
I'm writing about them because I only just got the nerve to buy one and try cooking it -- and it was a complete success. There are lots of different ways to do it, and I figured the most foolproof one would be boiling and mashing. The only difficult part is cutting it up beforehand. You'll need a non-serrated knife that is long, strong, and sharp, and a large sturdy cutting board. Then you'll have to pare the rind off the pieces you cut.
After that, it was like boiling any other root vegetable -- cut the pieces into a manageable size, cover them with water in a pot, put the lid on, and boil. I let them simmer for about half an hour, then drained them and mashed them. The flavour is something like a parsnip or a carrot, not quite as sweet as a beet. The texture is a bit more fibrous.
It really was tasty enough that it could have been eaten with no toppings whatsoever. I ended up mashing it with olive oil and then mixing in some grated cheese and chopped green onions. One recipe I read recommended sour cream. Purists seem to go for salt and pepper and butter. I'm thinking ahead to my herb garden later in the summer and imagining that rutabaga would be great with basil and rosemary.
Anyways, that's an easy and foolproof way to cook a big cheap ugly vegetable. Google around a bit and you will probably find recipes for roasting or baking them, and suggestions for serving them raw on veggie platters or in salads.