Onion sets are small, immature onions which will grow to full-size by the time of harvest.
Dig over the soil and turn in manure two or three months before planting. Do this in advance as the high nitrogen of fresh manure is not required by these plants. Improve drainage in heavier soils by adding grit and organic matter, and build up your raised beds.
The soil pH should be neutral to mildly alkaline, so use lime if it is acid. Plant out the sets in shallow drills, covering each bulb so that the neck is just protruding from the soil surface. Leave a spacing of around 15 cm between plants. If you have a raised bed, plant out two rows on each bed, 20 cm in from the side of the bed. Weed diligently and be sparing in your water regime.
Growing from seed, sow into modules in January or February for a summer harvest, or in the early autumn for an early harvest the next year. Use a special seed compost, and reckon on around five seeds in each module.
You won’t have to fiddle around separating out the plants as the bulbs will push themselves apart as they expand and grow. Don’t forget to label each tray with sowing date and variety.