You can grow onions and shallots from either seed or sets but in cooler damper areas (Yorkshire) you can expect a much more reliable yield from sets over seed grown onions.
To grow some really good onions some pre-planning is required. Planting onion sets into freshly manured beds is likely to cause the onion sets to rot also the high levels of nitrogen will encourage bolting (going to seed). To avoid this prepare your onion beds in late summer, ready for planting the following spring.
Choose a sunny spot with good drainage, mulch well with well rotted manure and dig in thoroughly then keep the patch weed free over the winter
Onion sets are best planted during February to early April when the ground is frost free, workable and not too wet. Don’t worry about frost after planting (not a problem). Plant the sets 6” apart in rows 12” across or if space is limited you can grow onion sets in a block or square, but give them 8” apart each way.
If you’re working with heavy soils or poor drainage you can lift a ridge of soil and plant the onion sets on top.
Plant the onion sets with just the tip showing out of the ground. Make sure they are pressed in firmly but there’s no need to hammer the ground solid.
Harvest late July-August when the leaves start to turn yellow. Watch the weather forecast for a dry spell, then lift and if possible leave on the ground to dry over. If it’s going to rain bring your onions into a dry shed, conservatory, garage etc to dry out. Do not cut the leaves or roots at this stage.
When the foliage is dry & yellow cut off 2” approx above the onion & store with lots of airy ventilation in a frost free shed, garage etc.