My brother in law, can grow anything, build anything & make money on anything he sets his mind to.
He plants onions in a large 20 gallion pot & let them go to seed, fall to the soil in the pot, then pull up the mature onions.
In spring he thin the young onion, the ones he remove are put in trays & transplanted into his garden.
He plants his elephant garlic(leek) under his single row of muscadine grape, he has about ten rows, 25 feet long.
I wait till my onions blooms start to turn brown, I cut the stem about an inch or two long & drop them in a dry brown paper bag for six weeks.
Six weeks later, or anytime after six weeks, roll down the top of the bag, like a lunch bag, tape it & shake it, rest 7 shake it again, open it.
Once open, remove the flower & look for seeds in the flower, it should not have any, if it does, wait another week.
If it is empty, remove each head & the little black seeds are the onion seeds, there will be bits /trash in seeds.
I know many people who plant rows of seed baring plants in rows, a seed tray width apart, then put down the seed tray when the time is right & the seeds fall on the soil & the tray is moved to the green house, if it is fall.