From an Extension pub. It's not mentioned here, but I think since it's a root vegetable, having a lighter soil helps. Good luck with your crop this year.

Starting onions:
Onions can be started three way: sets, direct seeding and transplanting. Though planting onion sets is the most popular way to grow them, you'll have better results transplanting seedlings you start indoors ahead of time.

Transplants:
Raising your own seedlings for transplants gives you a jump on spring in late winter, more choices of varieties to raise, plus it's less expensive than purchasing transplants. Transplants are available through mail order and some garden centers. Regardless of the source, transplants will give you bigger onions by fall. Onions need to be planted indoors 8 to 12 weeks before transplanting outside, so plant the seeds in February to transplant onion seedlings outside in May. Buy only as much seed as you will use this year. Onion seeds don't remain viable for much over a year.

The most important thing to remember when buying onion seed is day length, especially if you're ordering from a national mail order firm. In Minnesota, we need to buy "long day" onion seeds because our summer days can be 16 hours long.

In the south, day length varies less and averages about 12 hours of light per day. Onions for southern gardeners are "short day" onions. You can raise short day onions in Minnesota but they will bulb as soon as day length reaches 12 hours and you'll get small onions.

Long day varieties start to bulb when day lengths are about 14-16 hours. If you start onion seeds indoors, keep lights on only 12 hours each day to give the plants a suitable night. Onion seedlings will form bulbs too early if exposed to long days at any time during their development. You will not get anything bigger than sets.

Plant onion seeds ¼-½" deep in a sterile seed-starting media. They can be planted 3 to a cell in recycled four or six packs or sow thickly in rows if you plant in a deep (4" to 6") flat or other container. Keep tops trimmed 3-4" tall. Water regularly to maintain adequate soil moisture.

Onion seedlings need to be hardened off before transplanting outside, after danger of frost has passed. Expose young plants to outdoor conditions gradually, over a two week period.

Plant seedlings 4" apart and just deeply enough to catch in the soil. When mature, they'll appear to sit on top of the soil.

Once outdoors, keep onions well watered, about an inch a week. Watering is critical when the bulbs start to swell. Water stress produces stronger flavored bulbs. Add fertilizer, based on a soil test, at planting, then again when leaves are 6" tall and lastly when bulbs are starting to swell.

I started onions on March 1, and the seedlings have been up for around a little over three weeks now. They are long day, and they've been under 40W fluorescents for about 15-16 hours a day. Have I exposed them too long to get good bulb formation? Should I scrap them and start over?