I have also had this problem this year. White & red onions (planted from sets) especially so. Yellow (from sets) not so bad, and the Candy variety (from seedlings) did the best of the four.

I grew all of my onions in the same raised beds. It was a very hot & humid summer here in Vermont. I didn't water much after the plants were established, because we had enough rain showers.

I added a lot of dry composted cow manure to that garden bed, so I'm wondering if the soil was too rich. That might account for the tops being enormous. After I trimmed the tops, they grew right back in no time. Necks were very thick.

I let them dry on a rack in my garage for a couple of weeks. It's been like an oven and the garlic has dried nicely, but many of the onions are getting mushy.

And I had such an awesome crop of onions last year. I thought I had this onion thing figured out!

Hmmmm....