Various types of onion, including spring onions, pickling onion and shallots can be successfully grown from sets or from seed, some from both.
Onion sets are small, immature onions, planted in spring or late summer. The sets increase in size and each forms one full-sized bulb when ready to harvest. Where possible, choose onion sets that have been heat-treated. This means their flower embryos have been killed, so they're less likely to run to seed or bolt. Generally, growing onions from sets is easier and more reliable than from seed and in cooler, damper areas, the sets should give a better yield of larger bulbs than if grown from seed. However the range of varieties available is far greater if growing onions from seed.