possibly a fungal thing or onion fly damage. can you see any small maggots in the rotting onion bulbs.
next year grow them in a totally different part of the garden.

Onion Root Rot. This would seem to be like Brassica Club Root disease in so much as the virus remains in the soil for many years. So, you won't be able to grow onion, leeks and garlic for some time to come.

My first thought is over saturation & poor drainage. The soil should be loose, dark & fluffy. Try mixing in some silica or pure sand. Onions love that as most root vegetables do. Onions naturally retain lots of water. Let them dry out between waterings. Full sun - space them out.

Thanks for your advice. Yes there are small maggots in the root. I have been told to dig them up and burn them

Alan

The maggots are a secondary issue. They don't cause the onions to rot. They come because of it. Floating row covers can help as well as keeping your beds drier & warmer.

https://www.doityourself.com/stry/how-to-rid-your-garden-of-maggots-once-a-year