If there is anyone growing onions in your area, they would be the ones to ask. That said, I plant my seed the 1st of September. You can plant directly into the row and thin or transplant the too close ones into another row. However, it is easier to manage if you plant into a small prepared place and then transplant into the permanent rows.
I transplant mine into the rows Nov 15 before the fall rain starts. The day before, if I can manage it.
Temps here are winter-20 t0 28F, and summer () to 110F.
This fall, I intend to plant seed somewhat earlier as my transplants are somewhat smaller than the ones Dixondale
farms sell.

Here is a link that might be useful: growing onions in Southern California