1. Preheat oven to 425°. Scrub beets well, and pat them dry.
2. In a medium bowl, mix pepper, oil, 1 teaspoon salt, and coriander. Add beets, and toss to combine. Place beets on a large piece of aluminum foil, and wrap to cover, ensuring that beets stay in a single layer; close seams tightly to keep steam inside as the beets cook. (If you are using different colors of beets, wrap each color separately.) Discard remaining marinade. Place foil packet on a rimmed baking sheet, and bake 50 minutes or until cooked through. (Beets are ready when easily pierced with a knife.) Remove from oven, and let cool to room temperature.
3. Remove beets from foil. The skin should easily peel off with your fingers or a paring knife. To protect hands from getting stained, you may want to rub skin off with paper towels or wear gloves. (The spice marinade may have concentrated itself on parts of a beet. If so, gently scrape it off.) Cut beets into wedges, and arrange on 4 individual salad plates.
4. Place the yogurt, ginger, sugar, and 1/8 teaspoon salt in a bowl; whisk to combine. If you prefer a thinner dressing, add a little water. Drizzle a bit of dressing on beets, wait a few minutes, and then drizzle some more; the beets will absorb the dressing. Garnish with cilantro, and serve immediately.