Cursed cullinary tip #4983534: once you've opened an onion– as in, removed the papery outer layers that flake and get everywhere so only the good eating layers are left– it turns out you can cut them with a butterknife. (It's the very fine serration near the tip, I believe.)

I leave the problem of what to put between the onion and the countertop to you¹ but if your eating could be improved by hauling half a leftover roundboi out of the fridge and slicing/chopping a bit without having to make A Whole Entire Cutting Board & Actual Good Knife Production out of it... go forth!