In a large saucepan set over medium heat, cook the onion in the oil, until it begins to soften and become translucent.
Add the broth, sweet potatoes and tomatoes and bring to a boil, reduce heat and simmer, stirring occasionally, for 15 to 20 minutes. (Slow Cooker Directions: Alternatively toss the onions, broth, sweet potatoes and tomatoes into the slow cooker. Cover and cook on LOW until the vegetables are tender, 6 to 8 hours.)
Remove from the heat and allow to cool slightly. Then puree in a blender or food processor until smooth. (This may require two batches.)
Return the mixture to the saucepan or slow cooker. Stir in the orange juice and additional water, if the soup seems to thick. Season to taste with salt and pepper.