The care and cultivation of onions once they are in their final position and spacing is the same whether they have been grow from seed or from sets.
Although not greedy plants onions do benefit from the application of a general purpose fertilizer applied to the soil a few days before sowing or planting. If you want larger onions then use a high potash content fertilizer, however this reduces the amount of sulphur and so reduces the flavour of the onions. For smaller but more flavoursome onions use a low potash fertilizer. Personally I use a mix of pelleted chicken manure (for nitrogen) and blood, fish and bone meal.
Because onions produce tall, thin, hollow leaves they are susceptible to weed competition. To avoid damaging the crop the only real way of controlling the weeds is by hand. If you do hoe then do so very shallowly to avoid loosening the soil around the roots and bulbs.
Water regularly throughout the growing season, to avoid any check in growth. Once the bulbs start to ripen, as indicated by the yellowing and dying back of the leaves discontinue to water.
Harvesting and Storing Onions
Although you can lift an onion at any point for immediate use if you wish to store your onions for later use you must wait until the foliage is starting to bend over of its own accord. This is where one of the many contradictions in gardening comes in, you will read in many books about bending the foliage over, to encourage ripening, while others will tell you not do it because it causes damage to the neck causing problems in storage. It is a matter of personal choice and I tend to favour leaving them to their own devices but will give a helping hand to any that are being stubborn.
If possible wait for a dry day to lift you onions do this by gently easing a fork under the bulb and lifting slightly, leave for a few days and then lift completely. Once lifted lie them on the ground to ripen and dry out, turning them occasionally to ensure even ripening and drying. Alternatively place the onions on a rack outside so that the air can flow all round the bulb.
If the weather is wet you will need to provide some sort of shelter or bring the onions inside as long as it is cool, using a greenhouse is not recommended as the onions can get too hot and start to cook which will prevent them from keeping well.
Once they have dried out, string them up and hang in a cool dark place. Or they can be stored on trays of wire netting, or in nylon nets, however it is best to air circulating all round the bulbs. The storage place must be dry, airy and frost proof. Check the base of the bulbs occasionally for rot starting and remove those bulbs to prevent it spreading.