1. In a large pot, sauté the onions in butter and oil.
2. Add broth and lemon juice, bring to a boil.
3. Peel sweet potatoes and cut chunks directly into the pot, one at a time to avoid discolouration.
4. Reduce heat, simmer over medium/low heat for 20 minutes or until everything is tender.
5. Add black beans and pepper, cook for an additional 2 minutes.
6. Using a blender, puree the soup until smooth.
7. Serve hot, with a dollop of sour cream and sprinkle of parsley if desired.