Published on Organic Gardening (http://www.organicgardening.com)
Sow onion seed 8 weeks before the last spring frost to 17 weeks before the first fall frost. Seeds sprout in 7 to 12 days at 65° to 80 °F. Grow seedlings at 60° to 70°F.
Links:
[1] http://www.organicgardening.com/seed-starting-onion
[2] http://www.organicgardening.com/learn-and-grow/onions-growing-guide