Many thanks for your recent message regarding the rot which has unfortunately afflicted your show onions. If, as I suspect, your onions are suffering from onion white rot (Sclerotium cepivorum) then there is very little you can do. This fungal disease is capable of surviving in the soil for up to 15 years and there is no chemical treatment – the only course of action open to you is to burn the infected onions and not return to growing this crop, along with leeks and chives, for at least 8 years.
You could introduce some raised beds filled with new, imported top soil and plant your onions in these, but otherwise the outlook is rather bleak.
Sorry to be the bearer of bad news; hopefully this information will prove to be of some help and many thanks once again for your enquiry.