Haven't started my onion seeds yet. Just picked them up this Friday but they will be planted soon. I have a sun room -- stays at least 40 degrees in the coldest of our temperatures -- so I can start the onions out there without grow lights. They do take a bit of time to look like they are happy to be growing, but they will grow. And those early plantings will give you better onion plants when it's time to put them outside.
Once mine get over three inches tall, I take a pair of sharp scissors and give them an inch haircut. It seems to me that clipping them enourages new leaf growth and stronger plants much like pinching centers out of marigolds encourages bushy plants.