Edit—-.
 
as for mrs dash, i’ll repeat what i said:  i made a mistake; either i was confusing it with another product that does have msg/salt in it, or, they did, at some point, offer something that had msg/salt.  the current line of products does not have either.
 
as for the topic of the thread, the op asked about spicing the meat.  salt and pepper are all that are needed for flavoring the meat.  that’s the way it’s done here in philly – if it’s even added at all and most places don’t usually put s/p on while cooking the meat.  it’s not a purist thing at all, it’s just how it is 🙂
 
things like sauted peppers, onions, mushrooms, etc and cheese are extra’s added to the sandwich.  most places use these items to enhance the flavor the sandwich, which is the meat.   they’re also sometimes used to hide the fact that the quality of the meat is less than perfect.
 
buck & vi, you’re using a very good quality meat and should only need just a hint of salt/pepper to enhance it.  extra’s added to the sandwich should not be of such quantity as to hide/overwhelm the flavor of the beef. good quality beef that has no or very little gristle in it is key.  the meat should be tasty enough to not need any of the extras!
 
one thing you can do to add some bit of extra flavor, besides salt/pepper, is to use olive oil to saute the meat.  not many places do that here and it really makes a difference.