rotting onions

hello sonnycbr - sorry to hear you are having problems with stored onions, but it sounds like you've been doing the right things.  It could be any number of factors - whether you let them die back naturally, or did you harvest them while still green or on a wet day, ensuring they have dried thoroughly (I don't mean a day or two but a couple of weeks or more?), stored where air and light will get to them, and avoided cold damp conditions.

Some people leave both the old leaves and the roots on, but I will trim the roots back a little at the same time with a gentle thumb test to make sure the base is sound (I have white onion rot on my plot so each onion has to be inspected carefully   )  If you trim the dead leaves off, make sure you don't damage the top.  Avoid damaging the onion (eg dropping it) as bruising might start off rotting. 

A tip: always use the biggest up first and the smaller ones until last ... big ones tend to go "off" faster.  And finally, is the variety a good storer?  Some simply don't keep very well.

As mentioned, there is onion rot on my plot but I usually see the damage before storing.  Tell tale signs are white fluffy bits, and a sort of bruised brown staining in the white flesh by the base ... even a small bit spells doom and they cannot be stored.  Perhaps you've have some sort of (mild) onion rot infection where you've been growing them but it wasn't too obvious when you harvested the onions?

Hope you manage to find out the cause - there's nothing worse than finding all your hard work and harvest going to rot.