I make this about 3 times a month - it's very standard lunch fare at the Beck house.
1 tbsp vegetable oil
1 small onion, chopped
6 cups chicken stock
2 large sweet potatoes, peeled and chopped
1/2 tsp ground nutmeg
1/4 tsp black pepper
In a large saucepan, warm oil over medium-heat. Add onions and saute for 2 to 3 minutes or until onions are soft and translucent. Add chicken stock, sweet potatoes, nutmeg and pepper.
Cover and bring to a boil; reduce heat and simmer for 25 minutes or until potatoes are tender.
Remove soup from heat and, in batches, puree soup in a blender or food processor until smooth. Return soup to saucepan and heat to serving temperature.