Hardy from zones 3-9 onions, onions are a cold season crop. The easiest way to plant onions is from sets, and they can be planted outdoors a few weeks before the last frost (as long as it’s not below 20 degrees). You can see a full planting guide at The Old Farmer’s Almanac.
As far as planting goes, I’ve found a very short and helpful video of square foot planting onion sets to make the most of a raised bed:
And there you have it! Easy peasy.