Spring onions or shallots as they are sometimes known often run to seed when weather warms in spring.
Seed of green onions does not keep for long and seed collected for sowing next season will produce a vigorous crop as this seed will have come from plants that have adapted to your local soil and climate conditions. Leave several of your green onion plants to produce seed from their globular flower heads (umbrels). As seeds develop the umbrel appears like a globe of tiny greenish-white ‘buds’. As the seed matures the heads change to a pale grey and the buds begin to open and ripe, black seeds can be seen inside the buds. Not all the seeds ripen at the same time.
The usual method of collecting seed from these plants is to wait until most of the seed has ripened, then cut off the seed heads into a paper bag and leave the heads until the rest of the seed ripens and drops to the bottom of the bag. After all the seeds have been collected, they can be separated from the papery debris in the bag.
However, with this method, quite often the first seeds to ripen have dropped to the ground. I prefer to take a medium sized bowl or brown paper bag with me when I go to check the vege patch and while holding the bowl under each seed head, I give the seed head a gentle shake. The collected seed is then transferred to a labelled paper bag until all the seed has been collected. I get more seed with this method and it avoids having to separate the seed from the debris.