Heat 1/4 cup of the broth in a large soup pot over medium-high heat. Add the onion and curry powder and cook and stir for 2 minutes. Add the remaining 4 cups of broth, the sweet potatoes, and the water. Cook for 30 minutes, or until the sweet potatoes are tender. Remove from the heat and let cool slightly.

Transfer one-third of the sweet potato mixture to a blender and process until smooth. Repeat the procedure with the remaining sweet potato mixture, processing it in batches. Return the blended mixture to the pot and bring to a boil. Remove from the heat, add 1 cup of the yogurt, and stir until blended.

Top each serving with 1 tablespoon of the remaining yogurt and garnish with the optional cilantro.

Stored in a covered container in the refrigerator, leftover Curried Sweet Potato Soup will keep for up to 3 days.

PER SERVING: 177 calories; 1.4 g fat; 0.2 g saturated fat; 7.1% calories from fat; 0 mg cholesterol; 4.9 g protein; 37.5 g total carbohydrate; 12.9 g sugar; 4.5 g fiber; 770 mg sodium; 128 mg calcium; 1.8 mg iron; 25.4 mg vitamin C; 13266 mcg beta-carotene; 1.3 mg vitamin E