And yes, i would have tried all the rest before i picked onion. This is a very good tip and as counter intuitive as not putting those yams and sweert potatoes in your fridge. This is good news for those of us who love avocado and it has the benefit of also storing prepared onion as well.
I wonder what else behaves well. since you now have a container with a layer of onion it may be time to experiment. Try a cut apple. Who knows?
We are all been exposed to more and more exotic foods and that always means learning how to work with them. All of them have some skill and knowledge involved. Fortunately we have ample videos showing us how these days. Yet this trick is not likely out there quite yet..