How to Make It
Preheat the oven to 400°F. Line a rimmed sheet pan with aluminum foil.
Combine the potatoes, 1 1/2 teaspoons oil, and 1/4 teaspoon salt in a medium bowl and toss to coat. Turn the potatoes out onto half of the sheet pan, spreading them into a single layer. Add the beets to the bowl with another 1 1/2 teaspoons oil and another 1/4 teaspoon salt. Toss to combine. Turn the beets onto the other half of the sheet pan, spreading them out as well. (You can create a border by folding a sheet of aluminum foil and wedging it between the edges of the sheet pan crosswise to create two distinct sections; this helps keep the red beets from bleeding into the potatoes.)
Cover the pan with aluminum foil (you may need 2 sheets), crimping it around the edges of the pan to seal. Roast the vegetables for 20 minutes. Remove the sheet pan from the oven, uncover, and give the vegetables a stir (keeping them separate so the beets don’t stain the potatoes). Remove any potatoes that are tender. Wearing oven mitts, cover the pan again and continue to roast until all the vegetables are tender, 10 to 20 minutes. Remove from the oven and place the potatoes in one bowl and the beets in another bowl. Stir the dill into the potatoes and the chives into the beets. Set aside.
Combine the cucumbers, kefir, sour cream, onion, mint (if using), lemon juice, and 1 1/2 teaspoons salt in a blender; blend until smooth, adding ice water by the tablespoonful if necessary to thin to your desired consistency. Taste and add more salt if needed. (At this point the cucumber soup can be refrigerated in an airtight container for up to 1 day; for an extra-silky texture, strain the soup before refrigerating.)
Pour the soup into 4 bowls. Divide the warm potatoes and warm beets among the bowls, dollop with sour cream, and sprinkle with chives. Drizzle with the remaining oil and serve.
Adapted from Sheet Pan Suppers Meatless by Raquel Pelzel (Workman Publishing). Copyright © 2017. Photographs by Ken Carlson.