My onions have mildew what can I do to save them

Is it mildew or white rot? I don't know a lot about growing onions but I have heard that white rot stays in the land and you mention some allotments. If you want to grow onions there....they will need to be grown in pots.

If it's white rot, it will have a kind of white furry appearance. The onions won't keep and by keeping them you will spread the infection.
As Amblealice says, never try to grow onions, shallots, garlic or leeks on the same soil (or any other things of the allium family.) On an allotment, you could overcome the problem by making a deep bed or box ideally isolated from the soil beneath with a piece of weed suppressing membrane and fill it with fresh soil from a clean site or with potting compost. As onions are shallow rooted there shouldn't be any problem, but you will have to make sure you don't introduce fungal contamination by using tools with which you've dug the main allotment soil.