I made chili last night and it is way too oniony. I know you can use potatoes to make things less salty; any advice on making already made food 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.
Since it is already made, simmer the chili at low heat for another hour or so allowing the onions to cook further and soften. They will slowly dissolve into the stew and the oniony flavour won't be as harsh.
Generally all flavours will mellow out with cooking. (@citizen)
Adding a bit of sugar or agave nectar may cut the oniony taste as a last ditch effort.
Furthermore, if you don't like the oniony flavour that onions do tend to impart on dishes, try caramelizing onions instead of a quick saute or slow sweat. Caramelized onions will add a much sweeter, milder onion depth-of-flavour without the harsh (sometimes bitter) kick of raw onions.