I've had problems with storing cut onions in the fridge. They dry out, sprout, go moldy, or all of the above. I've tried a few methods. Cut side down in zip top bags? Dried out and partly sprouted or moldy. Cut side down in a glass storage container with a plastic lid? The same results. Those plastic onion keepers? I tried one. It didn't keep the onions terribly long and it cracked in half after a few hand-washings. There went my whole plastic onion keeper budget. Boo-hoo. What to do?
Cook's Illustrated tried methods I wouldn't even consider. Water? Oil? No. In the end, they came to the same conclusion as I had: Cut side down in plastic wrap.
This method worked but while it held off the onion's moldy beard, it didn't give my onions the legs I was seeking. I don't use onions often and frequently find myself needing half an onion to last.
So, after considering cheese storage options, I thought I'd try a layer of parchment paper inside the plastic wrap. It worked. Four weeks later and my onion was moist, free of mold and had only done a wee bit of sprouting. As the folks at CI found, it's not great for raw applications. It didn't taste metallic as their tasters found with onions stored in plastic wrap alone, it just hadn't retained full freshness. For cooking/sauteeing, however, it was dandy. Just to be sure nothing scary was growing, I did remove a thin slice from the cut side before dicing the remainder for use in a soup I'll be posting soon.
These results were with one-half of a white onion. Different onions have different amounts of moisture so I'll be trying this method with a red and yellow onion and will get back to you.