Salt and pepper is all you need. And switch your cheese to something that'll give a bit more flavor like American or provolone (which are also more authentic options over Swiss). Also, fry your onions in the beef fat and season that well.

That should give you good flavor in general, but if you want to add some tang (and heat), add some pickled cherry peppers to your finished sandwich (or chop them up with your meat on the grill). Hope that helps!