Place the olive oil, butter, and onion in a soup kettle, set over low heat and cook until the onion has softened, about 5 to 7 minutes. Increase the heat to medium, add the carrots and stir-cook for 1 minute. Add the celery and stir-cook for 1 minute. Add the green pepper and stir-cook for 1 minute. Add the diced potato, cumin, chili powder, and allspice; stir-cook for 1 minute. Add the herb bundle, lentils, and chicken broth; bring to a boil, then boil for 1 minute. Cover and simmer for about 1 hour, or until the lentils are tender. The soup may be prepared in advance up to this point.) Season the soup with salt and fresh pepper to taste and discard the parsley-bay leaf bundle. Ladle the soup into warm bowls and serve steaming hot. Makes 4 to 5 servings. [WASHINGTON POST JAN 11, 1989]