Presently no fix for white rot, although have a search and one or two universities are researching in to it, would make interesting reading if I understood what they meant in layman terms.
There are onion varieties that claim resistance, but resistance isn't immunity.
Mammoth seeds have a variety called Golden Bear of which they say:
Early maturing globe shaped with vigerous growth. Heavy yeilding. Highly tolerant to Downy Mildew and Botrytis. Resistant to White Rot.
50seeds £2.10
Again: Resistance isn't immunity
Sure I have read of another variety, as I seem to recall a different name mentioned.