Heat 1 tablespoon of olive oil and 1 tablespoon of butter in a small pot on medium heat. Place the yellow onion, celery, carrot and sausage into the pot and cook for 5 minutes on low heat. Add the chicken stock and black beans to the pot and bring to a simmer on medium-high heat. Add salt and pepper. Once soup is at a simmer, reduce heat to low and continue to simmer for half an hour. After the soup has simmered for 30 minutes, blend the soup in a blender for 5 minutes. Garnish with red bell pepper and sour cream. Divide the garnish equally among each portion.