I don't have a lot of experience here, but I'll offer an opinion. Before anything starts to rot, of course you can dehydrate. But mush dehydrates into powder, not into strands. Won't look like dehydrated onion slices. You can use unrotted onion mush to make edibles, so dehydrated unrotted onion mush can be used as well. Flavor is still there. It's the texture that has gone south.

You might try asking for advice on the cooking forum or harvest forum.

I keep my onions stored in the barn, letting them freeze and defrost as the weather dictates. When I know I'll need an onion, I bring it in the night before and it's fine. They usually keep well into March. My neighbor told me about this technique several years ago, and I didn't believe it could work until I found an onion in March i had left in the barn and it was perfect. All the ones in the house had rotted.

Thanks, everyone. While some of the onions were frozen solid at one point and then thawed, since they were translucent, I was able to slice them on my mandolin and get 30 or so of them in the dehydrator. The first batch is done and I have a nice bag of thin-sliced onions, and a second batch in the dehydrator.

I have 10 pounds on the stove cooking down for onion jam, and at least enough left for one more dehydrator load and maybe some onion soup!

More work than I planned on, and now I will have to pick up a 50# bag somewhere for the root cellar, but at least these won't go to waste.