Russian beet soup doesn't sound sexy, but this silky, lemon-herby version from Minneapolis' Chowgirls Killer Catering is shockingly good. (You can serve it any season, but the red hue is especially festive for Christmas!)
To make stock: Peel sweet potato, beets, carrots, onion and garlic cloves, reserving skins in a large saucepan. (If using golden beets, immediately submerge them in a bowl of water to prevent browning.) Add the 5 cups water to the pan with the skins, along with 2 whole garlic cloves, the turmeric, salt and pepper. Bring to a boil. Reduce heat and simmer, covered, for 30 minutes.
While stock is cooking, prepare vegetables. Chop sweet potato, beets, carrots and onion. (Again, submerge golden beets in a bowl of water, if using.) Mince the remaining 3 garlic cloves.
In a large skillet, cook the onion in hot oil over medium heat for 3 minutes. Add cabbage; cook 3 minutes more. Add minced garlic; cook 1 minute more. Remove from heat; set aside.
Remove stock from heat and strain through a fine-mesh sieve. Return 3 cups of the stock to the saucepan (reserve remaining stock). Add sweet potato, carrots and drained beets to stock in pan. Bring to a boil. Reduce heat and simmer, covered, until vegetables are tender, about 30 minutes. Remove from heat; let cool 15 minutes.
Transfer cabbage and onion mixture to a blender or food processor; add vegetable mixture. Cover; blend or process until very smooth, 5 to 6 minutes. Add 4 tablespoons lemon juice; blend for another 30 seconds. Add some reserved stock to reach desired consistency. To make soup extra smooth, pour through a fine-mesh sieve into a large mixing bowl, gently pressing soup through with a silicone spatula until all liquid is removed. Chill until very cold (or, if desired, serve hot).
Before serving, prepare garnish: In a small bowl, stir together yogurt with 1 to 2 tablespoons of the remaining lemon juice. Serve soup in small bowls or large shot glasses, garnished with yogurt mixture and tarragon.