Harvest Sweet Potato Soup

1 cup chopped celery
1/2 cup chopped onion
1 tablespoon vegetable oil
3 medium sweet potatoes (about 1 pound), peeled and cubed
3 cups chicken broth
1 bay leaf
1/2 teaspoon dried basil
1/4 teaspoon salt, optional

In a Dutch oven or soup kettle, sauté celery and onion in oil until tender. Add remaining ingredients; bring to a boil over medium heat. Reduce heat; simmer for 25-30 minutes or until tender. Discard the bay leaf. Cool slightly. In a blender or food processor, process soup in batches until smooth. Return all to pan and heat through.