A nice thought here Darren and for sure you can sow onions late summer, just a half inch or so apart, so they are only v small by year’s end, to re-plant as sets. But I recall the sowing date being critical, sometime in August – sow late enough that they are small and go dormant and then carry on growing when replanted, WITHOUT thinking they have done their "onion bulb" bit, which would prompt them to flower next spring. Sowing in early August might make bigger bulbs which would do that.
I think the best date is not before mid August but does anyone else know?