By John Merritt
That climate should be fine
I don't know where you live so I can't tell you when to plant and harvest but if your temperature doesn't go below -5 you should be good.
If everything goes ideally you could get up to 80 tons off 1 hectare but you'll probably get less than half of that at first.
In case you're still deciding on what type of onion to grow I'd recommend Walla Walla. They're sweet, big, hardy and will get more profit.
You'll need about 400,000 seeds per hectare. You want full sun and loose, just-barely-acidic soil without too much sulphur. 800 pounds of fertilizer should be good for 1 hectare.
Be careful with pesticide. Too much will kill pollinators. Which you need.
Also you should have a detailed plan for everything and a decent irrigation system or you could fail miserably.
- An onion farmer