Directions

1. Place beets in a medium saucepan, and cover with water by 2 inches. Bring to a boil, reduce heat, and simmer until tender, 30 to 35 minutes.
2. Remove beets with a slotted spoon, reserving cooking liquid. Let beets cool, about 20 minutes. Strain cooking liquid through a cheesecloth-lined sieve, and refrigerate, covered, until chilled, about 2 hours. Peel beets, and cut into 1/4-inch dice.
3. Whisk together stock, sour cream, yogurt, lemon juice, the salt, and 1 1/2 cups cooled beet liquid in a large bowl. Stir in beets, cucumber, and dill. Season with pepper. Cover, and refrigerate for 2 hours (or up to 1 day).
4. Stir soup, then divide among 8 bowls. Spoon 1 tablespoon tzatziki onto each serving, and top each with a dill sprig.