2Heat the oil in a large saucepan over medium heat until hot but not smoking. Add the onion; cook, stirring occasionally, until softened and just browned, about 7 minutes. Add the garlic; cook until fragrant, about 1 minute. Add the cumin, coriander, cayenne, and pepper; cook, stirring, until fragrant, about 1 minute.
3Add the tomatoes and salt; cook, scraping up any browned bits from the bottom of the pan, until the juices are released, about 2 minutes. Add the stock and 4 1/2 cups water (for a thinner consistency, add up to 5 cups water); bring to a boil. Add the beets and stems. Reduce heat; simmer until the beets are tender, about 35 minutes.