I would like to start some onion plants indoors. When should I sow the seeds?

Using a well-drained growing medium, sow onion seeds in flats approximately 8 to 10 weeks before you intend to plant them outdoors.  As soon as the seeds germinate, place the onion seedlings under artificial lighting.  An ordinary fluorescent shop fixture, containing one 40 watt cool white and one 40 watt warm white tube, works fine.  The light fixture should be placed no more than 4 to 6 inches above the seedlings.  Growing temperatures should be 55 to 65°F.  Insufficient light and temperatures above 65°F promotes spindly growth.  Also, thin the onion seedlings (within 2 to 3 weeks of germination) if the plants are crowded.  Harden or acclimate the onion seedlings outdoors for 7 to 10 days before planting into the garden.  Initially, place the plants in a shady, protected location.  Then gradually expose the plants to longer periods of sunlight.  Bring the seedlings back indoors if nighttime temperatures are forecast to drop to 32°F or below.  It’s usually safe to begin planting onion seedlings outdoors in early to mid-April in Iowa.