Preheat oven to 425 degrees F.
Wrap each beet individually in aluminum foil. Place on a baking sheet and roast for 30 to 60 minutes (time will vary depending on the size of your beets) until a knife inserted into the beet goes through smoothly.
Let cool slightly, then, when cool enough to handle, rub the beets with paper towels to remove the skins (they should slide right off).
Cut into slices or wedges, then place in a large mixing bowl to cool.
Meanwhile, heat the olive oil in a small frying pan set over medium heat. Add cumin seeds and cook for about 3 minutes or until fragrant and starting to pop.
Pour warm oil over the beets. Add onion, preserved lemon, lemon juice, and half of the dill and toss to combine. Season to taste with salt and pepper. Transfer to a serving bowl or divide among plates.
Mix together the yogurt and tahini. Dollop on top of beets in a few places and sprinkle with remaining dill and radish flowers (if using). Serve at room temperature.
The beet mixture will keep, refrigerated in an airtight container, up to a day ahead of time. Top with yogurt and fresh dill just before serving.
