There are many varieties of onion, and they can be classified by colour, shape, culinary use, and storage properties. I like to grow red varieties to eat raw in salads, and select other onion varieties with a good reputation for storing well to use in cooking during the winter months.
Buying onion seed offers the widest choice of varieties, but seeds can be hard to germinate. Many gardeners opt to grow from onion sets instead. Onion sets are immature onions, often heat treated to prevent them bolting to flower.
Onions mature in mid-summer. It is possible to overwinter onions by planting in the autumn, and doing so will result in an earlier crop. I have had mixed results with this method. It is possible to choose varieties that tolerate cold weather and mature faster if an early crop is desired.
Spring onions are even easier to grow than standard onions, with varieties maturing in 8 to 12 weeks. Sowing the onions in succession will allow picking from mid-year to the start of autumn, and there are even varieties that can be sown late summer to overwinter to produce an early spring harvest. Spring onions are ideal for growing in empty spaces on a plot, and for growing on a patio in pots and containers. They make a delicious addition to salads and sandwiches.