Hi Mike

I can see the virtue because if you haven't done it before and you take the
weight off when removing the lowest onion, they tend to slip down.
However if you have a lot, the knotting process will be tedious.
Try here (it suggests twist ties between)
http://www.thriftyfun.com/tf174436.tip.html
--
Jim S
Tyneside UK
http://www.jimscott.co.uk