Yam Soup With Yogurt

Ingredients

• 6 medium yams, about 2 pounds, peeled, cut into 1-inch chunks
• 1 red onion, peeled, coarsely chopped
• 1 small clove garlic, peeled, finely chopped
• 3 carrots, peeled, coarsely chopped
• 1 rib celery, coarsely chopped
• 1 bay leaf
• 1 tablespoon fresh thyme leaves
• 1 1/2 quarts chicken broth, homemade or low-sodium canned
• 1/2 teaspoon salt
• 1 teaspoon freshly ground pepper
• 6 tablespoons plain low-fat yogurt
• 6 sprigs flat-leaf parsley, stems removed, finely chopped