I like growing Onions because they are an easy crop. In a bad weather year, when growing vegetables is difficult, you can always rely on onions.
The easiest way to grow onions is from sets, which are tiny onion bulbs which have been treated to produce good bulbs. The treatment also reduces the possibility of the onion producing a flower and bolting. The best time to plant onions is mid to late March or April, depending on the spring weather when the soil is a bit drier and warmer. You can buy onions suitable for planting out in the autumn and over wintering, provided that your plot is well drained, ideally with some added organic matter. As with all bulbs, winter wet can cause the bulb to rot. To grow onions, the ground doesn't need to be really finely raked as it does for carrots and salad crops, just reasonable and not compacted.
Onions keep well so you can pick and use throughout the winter. Provided you store onion sets somewhere dry and cool, you can also carry over spares to the next gardening season. Onion sets are often sold in quite large quantities, 50+ it is helpful to know they will keep from year to year.
video How to plant and grow onions and garlic