Onions can be raised by seeds or be planting onion sets. Either can produce crops like this.
Growing onions from sets instead of seeds, is easier, but with one problem. Onions grown from sets instead of seeds can 'bolt' - run to seed - easier than those grown from seed.
The big advantage of growing from onion sets, is that the small onion sets - which resemble a normal onion, but in miniature - can be planted early in the spring regardless of weather conditions. no expensive heating of the greenhouse as with seeds.
Another bug PLUS is the fact that onions raised from onion set are less likely to suffer from the affects of an onion fly attack.
Onion sets can be planted out in early winter if mild and suitable conditions. Japanese Onion Sets are available in the Autumn and are ok planted to see the winter through - maturing as onions much earlier than seed or spring sown sets.
However, 'bolting' is a problem. but you can take steps to avoid bolting. You can use specially heat-treated sets (Don't try it yourself!) or use the smallest of the sets only! heat-treated sets have to planted a little later - mid-March - than the normal sets.
If you want to grow your onions from seed, then there is a greater selection of varieties available. Only a few varieties are available as onion sets.
If you have a heated greenhouse or an electric propagator, then the seed option is best, and they are not too difficult to grow.
Seed should be sown in temperatures of around 15-16C. No warmer than that, and can be grown cooler after germination to avoid leggy and week seedlings. They can be sown in seed boxes or pots, but better still in cello unit seed trays where they can be sown 3 or 4 to a unit.
Onion seed can be sown in the open ground from mid April, and will soon put on the growth needed. However, take care when thinning out, so as not to damage the remaining growing seedlings. If sowing in the open, sow thinly and cover with fleece or fine net to ward off the troublesome onion fly..