Overwintering onions requires the right climate. In the UK, we really are too warm, especially down here in Kent. In the Netherlands when I had an allotment there, we overwintered onions beautifully. You never realise how much you appreciate the gulf stream until you spend a while just across the water in the BeNeLux...
My approach these days is to start off onion from seed, usually around 1st of February, I go for varieties that have good keeping qualities. I plant them out, what ever the weather, without any hardening off, when they are around 6" tall. Drop them in a hole that is about 1½" deep, and firm them in well. At this time of year pigeons love to pull em out. Deep hole makes it harder. Water, and wait.
Usually they are ready to harvest in July, and if stored well will keep through the winter.
J