Hello Gina, yes mildew vexes me too but this year is much less present, so one probably could have grown the over-wintering onions. I think for mildew to develop a lot, it needs repeated rain in late May and June, which was absent this year thank goodness. In moisture at that time of year, any spores on the overwintered onions can breed. But spores also live in soil I fear so possibly, like a lot of growing, there is an element of gambling with the weather.
I grow White Lisbon spring onions, enough for salad onions and also some to bulb up in June. Sow late August, they resist mildew quite well.