A versatile yogurt yogurt sauce that could be used as a dip, a salad dressing, or as a schmear on a cracker or flatbread. The possibilities are endless.
Preheat the oven to 425 degrees F.
Rinse and clean the beets and trim tops and ends.
Place the beets in a baking dish in a single layer and fill the pan with ¼ inch water - just enough to create steam in the pan without simmering the beets. Lay a piece of parchment paper over the beets and cover the dish tightly with foil. Roast for an hour, or until completely tender when pierced with a paring knife.
Let the beets cool just enough so you can handle them, and then peel by rubbing with a paper towel. The skins will slip right off.
Coarsely grate the roased beets and stir into the yogurt.
Add the mint, tarragon, if using, olive oil, salt, and 1 teaspoon red wine vinegar. Stir and taste. Adjust salt and acid as needed.
Chill until serving. If desired, garnish with nigella seeds before serving.
Cover and refrigerate leftovers for up to 3 days.