Many of the seeds from nurseries and seed companies are hybrids, which are cross bred from 2 or more parent varieties. Hybrids should not be confused with GMOs, they are acceptable by organic gardening standards and have been common practice for many many years. However -seeds saved from hybrids will not necessarily produce a plant true to the variety they came from, they generally revert to the characteristics of one of the parent breeds. The germination rate of hybrids is also less reliable and at times seed companies will modify genetic composition of plant to produce sterile seeds - borderline GMO - genetically modified organism. Avoid harvesting seeds from hybrids.
Bulbs can be planted in mid-fall, but if you are planting for the purpose of gathering seeds, plant in early spring once the ground temperature reaches an average of at least 55 F. Onions produce circular clusters of minute white flowers known as umbrels in anticipation of pollination, they are self pollinating but cross pollination is not uncommon. If you are planning on harvesting the seeds to produce a particular variety or for specific traits of the parent plant - isloate it from potential sources of cross pollination, which includes other closely related alliums.
If you plan to save onion seeds from multiple generations be advised that Onions are prone to inbreeding deformities, like cousins breeding with cousins after several generations of self-pollination. Intermixing seeds from at least two different plants creates a more healthy genetic variation.
Harvesting Onion Seeds
When the flower clusters / umbrels start to brown it's time to harvest them. Snip the stalks a few inches below the flower head and place them in a paper bag. NOT a plastic bag, as this will lead to moisture build up and potential fungal issues and preferably not burlap or cloth as the seeds are very very tiny and will cling to and get lost in the fiber. Non waxed paper bags are best.
Place the seed bag in a dry and cool place to completely dry out. Once the flower heads have dried shake it up to coerce the seeds to seperate from the flowers. Completely dry flowers should yield clean seeds naturally, but if not don't worry about seperating the plant matter from the seeds - so long as it is fully dry it is harmless. Store the seeds in cool dry location over the winter.Another method of harvesting the seeds is to gently rub the dry flowers on a screen to free the seeds.
I've used both methods and find the paper bag method to be the easiest and most productive.
Onion Varieties Trouble Shooting Onions
Hydroponic Onions Onion Companions Walking Onions