Line a small casserole with aluminum foil, then roast several washed, unpeeled beets of uniform size, wrapped in foil, with the casserole's lid firmly on. Roast the beets slowly, at about 300 F. The length of time necessary will depend on the size of the beets, but it should take a good part of the day. The goal is to have the beets become very soft and caramelized inside. Peel them while still warm and slice about a half-inch thick. Toss in a dressing of three parts olive oil to one part balsamic vinegar. Serve on a bed of mache with the little rosettes left whole, either on individual plates or on a single platter. Drizzle any extra dressing over the greens.