5 or 6 medium red beets with greens (about 2 1/2 pounds with greens), stems and greens cut off and reserved 2 teaspoons canola oil 1 medium onion, halved lengthwise and cut into thin half-moons 1 tablespoon minced garlic 1 1/2 teaspoons ground cumin 1 teaspoon ground coriander Pinch of cayenne pepper (or to taste) 1/8 teaspoon freshly ground pepper 3 plum tomatoes, seeded and cut into 1/4-inch dice (about 1 1/4 cups) 3/4 teaspoon coarse salt 1 3/4 cups homemade or low-sodium store-bought chicken stock 1/3 cup low-fat yogurt

Cut beet greens into thin strips, and stems into 1/4-inch pieces; set both aside. Peel beets with a vegetable peeler; cut into 1/4-inch-thick matchsticks. Set aside.
Heat oil in a large saucepan over medium heat until hot but not smoking. Add onion; cook, stirring occasionally, until softened and just browned, about 7 minutes. Add garlic; cook until fragrant, about 1 minute. Add cumin, coriander, cayenne, and pepper; cook, stirring, until fragrant, about 1 minute.
Add tomatoes and salt; cook, scraping up any browned bits from the bottom of the pan, until juices are released, about 2 minutes. Add stock and 4 1/2 cups water (for a thinner consistency, add up to 5 cups water); bring to a boil. Add beets and stems. Reduce heat; simmer until beets are tender, about 35 minutes.
Add greens; cook until just tender, about 5 minutes. Divide soup among six bowls; divide yogurt among servings.