Sweet Potato Soup

2 tsp. butter
1/2 c. sliced carrots
1/2 sliced celery
1 large can chopped tomatoes
1 box Swanson beef broth
1 1/2 lbs. sweet potatoes, peeled and thinly sliced
pepper
low fat yogurt, for garnish

In a large saucepan, melt butter, add carrots and celery, saute for 5 min. stirring often. Do not brown. Add tomatoes and cook for 5 more min. Add broth and sweet potatoes. Bring to a boil, reduce heat, cover and simmer for 25 min. Puree. Add pepper and serve with yogurt.