What you're going to want to do is harden them off first. Going from indoors under artificial light to the great outdoors under the direct sunlight is too much for them, they must be gradually acclimatized. I start with about 20 minutes of direct sunlight the first day, or about 30 mins if its overcast. Continue with that for two more days or so then up the time to 40 mins or so. If its very windy or cold make sure you pick a sheltered location. Bring them back inside for the remainder of the time under your lights. Once they are managing about 2.5 to 3 hours of direct sunlight in the open after about two weeks you can plant them out in a sunny location in your garden. You need only cover the roots and then lightly water them, onions grow on the surface and do not need to be hilled up like leeks do and they do not like to be buried too deeply. During this hardening off process you should also reduce the frequency of watering and give them a light feeding of N fertilizer.
In regards to your question on lights, if they are a intermediate to long day onion type you should have them under lights for no more then about 13 hours and make sure their night time is complete darkness. You can strengthen the plants early in their life when they are under your lights by having a fan on them during the day, this simulates the wind and will strengthen their leaves. When done properly you do not need to trim them at all because this reduces yield. I never trim my onions leaves or roots at all because it reduces yield. Although most people will tell you this must be done it is simply not true. Its really just a matter of choice, nurseries do trim the tops because it makes them easier to handle and more uniform looking.
Long reply but I hope it helps.
msg me if you have any more questions.

-Ken

Thanks for the info.