My mother in law ate a roasted turnip at my house the other day. It was unfamiliar enough to her in that form (she'd had mashed turnips before) that she had to ask me what it was, and it was a reminder of the fact that this time of year truly is the only time that many Americans come in contact with the lesser-known root vegetables. While carrots, potatoes and onions are part of our daily lives, and sweet potatoes and beets are at least intermittenly familiar (if commonly hated), few Americans know celeriac, turnips, parsnips, taro, rutabagas, yams, jerusalem artichokes or many others well…