In a 2-quart pot, heat broth to boiling.
Cook sweet potatoes in broth until tender, about 20 minutes.
Drain; reserving the broth.
In a medium sauté pan, heat the remaining 1/4 cup broth over medium-high setting until bubbly.
Sauté onion until soft, about 5 minutes.
Stir in tomatoes and cook another 5 minutes.
In a blender, place onion mixture and sweet potatoes, adding just enough of reserved broth to puree.
Return potato mixture and remaining broth to stockpot.
Reheat gently, uncovered over medium-low heat until hot.
Garnish with chopped cilantro. (optional)