In a heavy saucepan, melt the butter/margarine. Add onions and cook until they are golden, do not brown.
Add the undiluted beef broth, 1 soup can of water, Coca-Cola, salt, vinegar, and pepper. If the beef taste is not quite there you might want to add some beef boulon to this. Cover and simmer 20 to 25 minutes.
In deep bowls add croutons, ladle soup and top with swiss cheese and sprinkel on Parmesan cheese.