I’ve been growing and planting my own seed for, well, a long time. Mine are a mix of whatever open pollinated keeper varieties have been available (not many) and selected over the many years. But I think the main variety is likely New York Early, a yellow. As many of you do I start them about now, in flats (mine are about 4" deep), broadcast or in rows depedning on how I’m feeling that day (widish rows are the easiest to manage). Since my seed varies in quality (we’re on the edge for growing onion seed) I usually plant thick and don’t thin enough. But even when crowded they pull apart easily enough. Keeping them thinned is best though (I tell myself that every year).
I’ve planted in the fall, mulched, and they generally do make it through winter but then they start growing too early and get frozen out. If they were in a cold frame that would probably work. For me it’s more reliable inside but a milder climate seems like it would work. I’ve also planted early spring and gotten a crop though they’re small.
A way around having to start seed is to grow your own sets. I’ve been doing this more now. In spring I plant either broadcast or wide rows seed fairly thick with a light mulch. Throughout the summer I pull some for salads. In the fall when they’re died down pull them (or rather scoop up a handful and sort out the bulbs), toss the smallest aside (and they’ll likely root, survive winter with no care, and grow next year!), cure as you do regular crop. Sort them for the sizes you like for sets (I go for about half inch), toss the smalls, eat the larger ones (or plant them next spring for quick greens).
Then I worry come spring that I’ll have enough and buy some generic sets from whereever and marvel at the low quality. Then stick them wherever I have a spot, and later marvel at how much i prefer my own.
When I harvest my main crop in the fall I pick out 6 - 8 of the best looking bulbs to replant to grow seed the next season. Unfortunately, onion seed is viable for only a few years, three seems to be max. Sue