February and early March are the perfect time to sow onion seeds indoors under lights here in Michigan. Growing onions from seeds rather than from sets gives you a lot more varieties to choose from as well as saving you quite a bit of money. Just make sure you choose a long day or and intermediate day variety if you are in zones 5-6.
Long Day Onions
Red Wing, Red Hawk, Bridger, Ailsa Craig, Cortland, Pontiac
Intermediate Day Onions
Walla Walla, Valencia, New York Early, Cabernet