I am trying to over winter onions for the first time this year as well. Everything I have read says you need to sow seeds early enough to get some growth before the cold dark days of winter. But not too early as you dont want too much growth or they will bolt in spring. So I am sowing three times, each 2 weeks apart and I will see which performs the best. First sowing was yesterday and the last will be first week of September. My garden gets full sun(when its sunny!) with no obstructions so I can usually get a way with a slightly later start for most veggies.

3 slowings is the way to go aug 1,15 & sept 1 just when I think I have the sowing time figured out the weather surprises me(like last year!) and none of my over wintering onions worked! but I always got over wintering onions when I did 3 sowings. Put the seeds directly in the garden, space about 1" apart to start out, you want to thin them to about 4" apart once they are about the thickness of a pencil. If they "don't work" and bulb up, you will at least probably get to use them like green onions before they flower and die.