Contrary to what you may believe, it is possible to put anything on a pizza. You'll call it sacrilege, blasphemy, a desecration, and affront to everything you hold dear about pizza, but dressing it in mustard isn't an offense punishable under the law. So you're going to have to deal with the fact that stuff like this happens occasionally.
Thanks to the NYC pizzeria Lions & Tigers & Squares, the internet has renewed reason to scream at each other because of pizza. On Twitter, Insider highlighted one of the restaurant's pies, which substitutes tomato sauce with mustard. Predictably, everyone with nothing better to do decided to flood Insider's mentions, posting multiple pleas to call the cops on such an outrageous culinary travesty.