In a large stock pot, salute onion in oil over high heat, until wilted and started to turn golden.
Add vegetable stock and sweet potatoes. Bring stock to a boil, reduce heat, cover and simmer about 20-30 minutes, or until sweet potatoes are tender.
Add chipotle peppers.
Puree the soup with an immersion blender, or in batches in a regular blender.
Add black beans, and salt to taste.
Add cilantro just before serving.
If desired, garnish with avocado pieces.