Remove the sausage casing. Heat olive oil in the soup pot and add sausage, stirring to break up the meat into small crumbles. Brown the meat and remove some of the sausage fat by blotting with a paper towel. Add onion, celery, and garlic. Saute briefly. Add beans, chicken broth, bay leaf, and tomatoes. Heat thoroughly. Just before serving, add the escarole or spinach. It doesn't have to cook--just wilt and stay a vibrant green.