Heat 1 tablespoon olive oil over medium heat in a large (5 to 8-quart) pot. Add onion, carrots, celery and saute, (cook while stirring occasionally) 5 to 6 minutes. Add garlic, parsley, thyme and rosemary and continue to saute until vegetables are softened, about 2 minutes more, adding more olive oil if pan gets dry. Add reserved beans, chicken broth, salt and pepper, increase heat and bring to boil. Reduce heat and simmer, partially covered (see NOTE), 45 to 60 minutes, until beans are just tender. Add sweet potatoes and continue to simmer, partially covered, until potatoes and beans are very tender, about 30 minutes more. Using an imersion blender, puree soup slightly, just to give the broth some body but still leave some chunky beans and vegetables. If you do not have an immersion blender, you can remove a cup or two of soup and puree in a regular blender or food processor, then return the pureed portion to the pot and stir into the soup. Stir in lemon juice.