Last year, I grew Buntons show stopper from seed. They all suffered from various levels of rot, even the ones in the greenhouse that , as far as I know, has never had onions grown in it before.
I am trying a mixture of seeds and sets this year to see if there is any difference.
So far, I have no onion rot in my garden, so if the worse comes to the worse, I will be able to grow a few there each year. Walla Walla onions did really well there, even though the 4 inches of home made compost is on top of almost solid clay.
cj :)