Heat the oil in a large saucepan over medium high heat and add onions, garlic, chipotle pepper and saute until onions are translucent, about 5 minutes. Add salt, avocados and stock. Bring to a boil then reduce heat and simmer for 10 to 15 minutes or until avocados are soft. Add sour cream or yogurt and simmer 1 minute more. Remove from heat and puree in a blender or with an immersion blender. Add more stock if too thick. Stir in cilantro and lime juice. Ladle into bowls and top with scallions.