KG, I have read about the tops dying back as well...but this is the advice I am using:
https://cru.cahe.wsu.edu/CEPublications/ ... FS097E.pdfHarvest and Storage
Dry onions grown from sets take three to four months
to complete their growth. You will know they’re ready
to harvest when most of the necks and tops have fallen
over. Once this happens, the onions will not get any larger
and should be pulled and allowed to dry in a shaded area
with good air circulation. After the outer skin becomes dry
and crispy, they can be stored in mesh bags in a cool, dry
location. This should only take a few days if the weather is
warm and dry. Sweet onions do not store well, but yellow
onions, followed by red and white types, are best for storage.
Never try to encourage bulbing by knocking over the tops
of onions yourself. This won’t stimulate bulb development
or growth and can bruise the neck and lead to rot.