Hi Wayne,
Yes, it is possible to harvest seeds from your single onion flowerstalk. Harvest time is when the first capsules start to open and show the black seeds. Keep seedstalks dry for the seeds to fully mature in the capsules before threshing after a month or so. A paperbag will be handy. You are likely to harvest some good and healthy seeds for next years crop.

But if you want to harvest your own seeds for more than one or two years, more than 20 individual onionplants have to flower side by side. Onions are known to suffer from inbreeding depression.

Sören.

I was going to ask the same question but I am afraid I do not understand the answer above. What do you mean by keep seed stalks dry? (they are outside subject to rain) what is threshing? Could someone explain in more detail Thanks

I've only done it for leeks so far but the onions seem to be following the same pattern this year so I think this will apply.

The onion puts out a flower stalk that ends in a capsule. The capsule looks much like a larger version of the end of a garlic scape but instead of bulbils, it'll go to seed. This opens up and puts out a compound flower (1 of my 4 "experiments" are flowering and the others are in the capsule stage right now). Once the onion flowers, you're left with a nicely spread flower head of maturing onion seed that looks like lots of dark colored dots. Around this time, you can cut off the stalks, put them in a paper bag - head first - and tie off the end of the bag or you can wait for the heads to dry down a bit and do the same thing. Hang the bags in a nice, airy, dry location for a couple of weeks to let them get really dry and then you're ready for threshing. Simply put, threshing is merely separating the seed from the rest of the plant. If you've bagged the seed heads, just shake them back and forth vigorously for a minute or so and all the seeds will have dropped to the bottom of the bag along with a little other dried stuff. Depending on how much chaff (non-seed) is in the bag, you may just be able to dump the bag into a sieve and let the clean seed fall through or shake in a bowl and pick the chaff off the top.

I hope I've helped rather than confused further ...