Spiraled in a baking dish, root vegetables form the base of a delicious gratin that can be served as fittingly for Thanksgiving or any holiday as it can be for an indulgent weeknight dinner.  Potatoes give a familiar and comforting flavor while beets, turnips, rutabaga, and other varieties lend a bit more interest, with the choice of customization completely yours.  Red beets are particularly delicious, though they do cause the entire dish to become quite tinged with their color.  For this reason, you may wish to opt for the golden variety. As a holiday dish, this one disappears quickly and can take the place of mashed potatoes if you are…