You have options for planting. Do you want to direct seed or grow transplants? If direct seeding you might find pelleted seed easier to germinate but if you're growing transplants it shouldn't make any difference.
Mostly I say this because if you want to buy pelleted overwintering onion seed you may have to buy a minimum of 1/2 million seeds. I'm not sure if this is in you budget. If it is, try Logan Zenner seed company.

I buy raw OW onion seed and grow transplants. My favorite varieties are T-420 (yellow) and Desert Sunrise (red), available from Osborne seed co. Johnny's sells Bridger, and Territorial sells High Keeper/Top Keeper. All these varieties are fairly similar in my experience with the 420 showing some more mildew resistance.

As far as seeding dates, again it depends on your planting technique. Maybe someone from your zone will have some specific seeding dates to share.

-Mark

I am direct-seeding them in early August, when I start seed-sown fall crops (leafy greens, carrots, turnips). Thinning rather than transplanting seems to work best. These varieties also make great scallions, so I gradually pull the little ones so the plants go into winter at the right spacing. When winter starts and I put on a tunnel, the plants are still small, maybe 8 inches, but they start growing in March and are huge by the time they bulb in May.

Desert Sunrise is a darling, flat little red onions once they cure, but Bridger is worth trying, too. Not a single plant bolted, and they are curing beautifully. Top Keeper is capable of getting really big, but I had a few bolters this year. I plan to try T-420 next.

Awesome. Thank you so much for the info. :)

Question on desert sunrise, Johnnys says adaptation is latitude 30-36. We are at 40, would these still perform well?

I'm north of 45 degree in NW Oregon and Desert Sunrise does great for me when overwintered.
Im not sure if the latitude they recommend only applies for spring planting but I have no problem with them and think they should work for you.

Thank you Mark, I think I will try the desert sunrise :)