Get the ground ready for sowing, on a dry day in March, sow your onions together with Lettuce, Beetroot, Carrots etc.

I'm a great fan of growing in modules to get onions going well early on and then getting nice large onions when planted out.
Sow two or three seeds in each 'cell' of the module and cover the seed with a sprinkle of compost, sand, vermiculite or whatever. Water with a spray or fine rose of a watering can. Cover the module with a sheet of plastic and place in a warm place but out of direct sunshine. Onions normally take 14 days or so to germinate.
As soon as they germinate, remove the cover. Keep the module well watered by standing it in a watering tray so the module can suck up water from the bottom.
As soon as the plants are about four or five inches high, just plant the modules out at the right distance (more space equals bigger onions) Multiply sown onions will grow just like ordinary onions but you should leave more space between each set of plants if three or more plants have come up in each module.
This takes all the uncertainty out of sowing in the ground and waiting for them to come up, surrounded by weeds and being dug up by cats etc.

Thanks Doctorbob1.

Bertiefox, i have been looking into growing them in cells as this method appeals to me too. What is the best time to sow them using this method? Will my north facing window sill be ok place the pots?