Making chili less oniony
Make another batch of chili with no or reduced onions, and combine them--freeze if needed, so you can use it all.  This is the only reasonly guaranteed way to resolve the issue.  Chili freezes and holds very well, so this might be practical even though somewhat extravagent with the ingredients.

I know that isn't what you want to hear, but it is the truth.  BTW, the potato and salt reduction thing is a kitchen myth.