If the plant isn't too woody at the base then you could try splitting now. put a spade between the leaves and cut down. you may loose a few shoots but the bulk of the two halves will be fine. if these are still too big repeat with each half. any small shoots that come away pot them up into compost and water well. if they are going to root they will do so in about 1 month.

I agree, now is the best time. I would pull it out of it's pot, then slice through it like a cake. I'd use a bread knife or something finer than a spade. Plant each piece as a separate plant. You can half or quarter it.

Thanks.by the way,last year I had spring onions that had dried into little sets ,I planted them again am now enjoying thumb size Spring onions.thanks again for that advice.