To start onion seedlings indoors in February or March, fill a container nearly to the top with a seed starting mix and then make two furrows, about 1/2-inch deep, for the onion seeds. Sprinkle seeds in the furrows and cover them lightly with more soil mix. Unlike onion seedlings grown indoors under grow lights, there’s no need to harden-off winter sown onion seeds because they’ve been outdoors from the start. Planting onion seeds by winter sowing means the plants are subjected to the natural day-night cycle right from the time of their germination. The tops of my florescent plant lights are flat, and they give off just the right amount of bottom heat needed to help onion seeds germinate quickly. Onion germination is fastest as 68-77?F (20-25?C), with slight temperature drops at night.