In Montana you are stuck with long-day varieties or a few day-neutral varieties. Of them Walla-Walla is sure going to be the sweetest (sure wish I could grow them) but as you have discovered they sure don't store. Alisa Craig has the same problem - short storage.

Texas Super Sweet is the best I can do down here for sweet and storage. Have you tried Yellow Sweet Spanish (4 mo. storage potential) or Highlander (same 4-6 mo)? Copra and Big daddy are the longest storing long-day varieties I know of but they have a bite and sure ain't sweet. :)

Have you considered alternate form of storage? ie: dehydrating them? I buy shipped in Walla Wallas when I can get them, chop and dehydrate them. Works great. Intensifies the flavor and will store almost indefinitely.

Dave

Many people chop and freeze onions in vacuum-sealed bags. Search Youtube for how-to videos.

There are no sweet storage onions. They are all sweet once cooked but hot when raw.

As I understand SULFUR is what makes onions strong. We also know that sulfur is a disinfectant and preservative.

So there is a trade of. as they say, You cannot have your cake and eat it too.