Ketchup, mustard, sweet relish or full length pickle spear.
It's illegal to put ketchup on hot dogs in Chicago, which is why I make sure to do it.
For real, people actually get offended. They don't pretend to be offended as a joke, they actually get offended.
In Chicago, you'd probably **** off fewer people by yelling "Heil Hitler" than you would by putting ketchup on a hot dog.