Preheat the oven to 400°F. Spread 1 cup salt over the bottom of a baking dish and sprinkle leaves from 1 thyme sprig over it. Place the root vegetables on top, sprinkle with leaves from the remaining 4 thyme sprigs, then blanket heavily with the remaining salt. Roast for 45 to 55 minutes. When cool enough to handle, remove the vegetables from the salt, dusting off any residual salt, then carefully remove any tough skin from the beets, rutabagas, and turnips. Leave the skin on the carrots and parsnips. Cut the vegetables into bite-size pieces.