The sweetness is twofold here: The beets caramelize during baking, becoming wonderfully sweet, and the slight sweetness of the balsamic vinegar then underscores them.
2bunchesbeets (10 to 12 small-medium)
Salt and freshly ground black pepper, to taste
Preheat the oven to 400°F. Trim off the leafy tops of the beets, but leave about an inch of the stem attached. Wash the beets well, scrubbing them with a vegetable brush, but leave them unpeeled.
Tear off large sheets of aluminum foil. Place 2 or 3 beets on each piece and gather the foil up and around the beets. Crimp together, sealing well. Repeat, forming several foil packages; place on a baking sheet.
Bake until the beets are tender, 1 to 1½ hours. (To test, slide the baking sheet out of the oven, carefully open one of the packages—stand away from the steam—and poke a beet with a paring knife.) When done, remove from the oven and partially open the foil packages. Let the beets cool to room temperature.
With a paring knife, pull the skins off the beets and discard. Slice the beets, letting them fall into a soufflé dish or serving bowl. Add the balsamic vinegar and a sprinkling of salt and pepper. Toss gently with 2 soup spoons and let marinate for at least an hour at room temperature, stirring from time to time. Serve at cool room temperature.