Arts said:

Spoiler alert: the onion has to be on the bottom so it doesn’t spill ad cause a slip hazard

I would like to see their testing.

This is a fundamentally misguided strategy. They really should be encouraging people to leave out the onion. If that doesn’t work, use onion powder. If that doesn’t cut the mustard, increase the size of the napkin. If that doesn’t work, use signs, reminders, and teach people to keep the onion on the bread. Only when all of these measures fails, should one consider requiring shoppers to wear non-slip footwear.