Funny you should ask, Gerry !
First, I would just say that I have often had onion plants sprout back up in the spring -- ones that I missed pulling somehow, even ones that were leftover onion seedlings that I started, never got around to planting because I couldn't find room, and just threw out on the garden in the fall. Those were all unintentional, but nonetheless made nice green onions in the spring; most bolted after a while and so didn't make too much of a bulb, but some did, if left to grow long enough. That didn't usually happen because they were in a place where I wanted to plant something else so they got pulled out.
In the spring of 2014 I had the idea of buying more onion sets than I needed (I always start a lot of onion plants from seed, but until this year - 2015 - I've always felt the need to kind of hedge my bets by planting sets as well), store them in the crisper drawer of the fridge, and plant them in the hope of harvesting some onions in the fall. So, on Sept. 7th I planted the sets, and they promptly started growing.
It took a while, but I finally came to the realization that those onions were never possibly going to bulb up, because they were long-day types; I harvested some to use for scallions, then as the weather got colder I set up a 'low tunnel' of plastic over PVC hoops over them. I continued to harvest scallions until the winter got really settled in, and then just let them be until spring. Once I could get back into that area I saw that the onions had apparently survived, and as the weather started to get a bit warmer (probably in March) I removed the plastic and just let them do their own thing. Had onions to pull for scallions, and then saw that the plants were beginning to form bulbs so I left them alone again for a while. I finally pulled all the ones that were left in mid July, and while probably about 1/3 or so of the plants had bolted and did not make nice onions, many of them had nice large bulbs and small "necks," so suitable for storing.
So... now that you've had the long version, the short answer is: Yes, onions absolutely will survive winter in zone 5 or colder! (I'm in the southern part of the upper peninsula, which was previously zone 4b but was changed to 5a when the USDA revised the zone chart in the fairly recent past.)
I don't think they really would need the low tunnel covering... but if you want to harvest some green onions as long as possible and then again as soon as possible in the spring, it makes them a lot more accessible.
