you are in the day long region. you can grow either day long or day neutral onions. if you are going to do this start immediately! i start mine on 2/22. i put mine out about 4/25. the goal is to have 5-6 weeks growth before planting out and then as much time as possible to grow as big a plant as possible before mid june. bulbing is triggered by the longest days in june.
how to start onions - put promix in container that's 1-2" deep. scatter or place seeds in rows 3/8" apart from each other in the row and the rows from each other row. i prefer to do the latter tho just sprinkling them on the promix works too. i wet the promix before putting the seeds in so when i cover them and mist/water the promix the seeds won't move. they come up in about 1 week, place under shop lights or in a full sun window. water as they dry out, fertilize with weak fertilier 2-3 times max before putting out, trim the greens when they are 4-5" tall back to 2-3".
onions are easy to grow, full sun, 1" water per week, don't compete well with weeds, fertilze just 1 time per month (may, june, july) and lose soil. come august when the tops start to lose their green color and some start to fall over, use a bow rake to gently push all the tops down, withhold water at this point, 1 week later pull them. for storage you have to cure them (if interested i'll describe) otherwise just store them in a cool place and eat.
sets are not worth the space, plants are ok seeds are the best. sweet (not hot) onions have thick necks and store just 2-4 months the hotter the onion the thinner the neck and the longer the storage.
tom