Billy By is a French cream of mussel soup cooked in white wine with onions, parsley and celery. It is served with cream and Parmesan cheese, either piping hot or ice cold. It was originally created for a customer at Maxim's without the mussels included in the final soup. Now they usually are included, although they may be served separately. No-one seems quite to agree on the source of the name. However, the most likely seems to be that Louis Barthe, chef at Maxim's, named it after a regular client, an American tin tycoon, William B (Billy B) Leeds, who always chose to eat the soup.