|I generally only use white onions, but I'm sure it works for all onions.
Anyway, a few months ago I bought myself an actual onion. I can't remember what recipe I was making, but it called for chopped onion and I didn't think the dried stuff would cut it. But after slicing off the little bit I needed, I felt like I totally wasted an onion!
Enter my glass Pyrex containers. If you use plastic tupperware/gladware/etc., then you aren't really living. Glass is the way to go! I received a set for Christmas a year or two ago and I can't believe I had gone my whole adult life without them! You can use them for storage or cooking, cold food, hot food, non-food storage, you name it!
So I put my leftover onion in one of the small Pyrex containers. It lasted for a good month while I slowly chipped away at it when I was cooking different dishes. By the time I used the last of it, it looked and felt as fresh as when I put it in the container! So there's my hack, lol. Instead of running to the store every time I needed one onion or having to waste onion, I put it in a small, air tight container and I can have fresh onion to use for weeks!
Okay, so am I dumb for going this long with figuring it out or are you surprised you hadn't thought of it before?