The beauty of Five Guys is you can have it exactly how you want. As above, be warned a 'regular' is 2 patties so is plenty big enough.So for those regulars of Five Guys, make me a couple of recommendations!
Personally I go with something like fried onions, gherkins, mustard, mayo and ketchup. Nothing else to get in the way. Lush.
So, just study the menu before you go and make it exactly what you want. Ain't more complicated than that.