Slow-Cooker French Onion Shredded Beef Sandwiches

Amazingly tasty.  Sherry vinegar is not available at my regular grocer but we have a “foodie” grocer that carries three or four varieties. This is NOT sherry cooking wine.  Do make the effort to find it.  The Slow-Cooker French Onion Shredded Beef Sandwiches are okay without the parsley but not as pretty.  Regular parsley works, too, though.