In a large saucepan, heat the oil over medium heat. Add the onion, celery, and garlic and cook, stirring, for 5 minutes.
Add the beets, potato, water, wine, parsley, thyme, salt and pepper, and bring to a simmer. Cook uncovered over medium-low heat, stirring occasionally, until the beets are tender, about 35 to 40 minutes.
Stir in the watercress and cook for 5 minutes more.
Remove from heat and let the soup stand for about 10 minutes. Transfer the soup to a blender or food processor fitted with a steel blade and puree.
Return to the pan and keep hot, or refrigerate and reheat later.
To serve, ladle the soup into bowls and spoon a dollop of yogurt into the center. Garnish with any extra sprigs of watercress.
Recipe submitted by Daisy on 3/8/00
