Spanish onions

You can buy varieties that are sweeter........but I don't think you can get them as sets. You'd have to grow from seed.

I'm growing Walla Walla Sweets this year (first time from seed).....I'll let you know how they turn out

John's onion page has this to say

The strength of an onion is due to the sulphur content and how you grow your onions makes a difference to how much sulphur they will contain.
Although, like all plants, onions require potash, the more potash they have available the less sulphur they absorb. Using low potash fertilisers will result in smaller, but stronger tasting onions.

so feeding and the soil they are growing in will make a difference as well

If you were a serviceman in Hong Kong in the 80s, and if after sampling the 'delights' of Wanchai    you found you had missed the last Star Ferry back to 'Kowloonside',  you could leap on a small bumboat to cross the harbour.  

These went by the name Walla Wallas.  I've tried to find out why that name but no joy so far!   :blink:   Cheers,   Tony.

They have Snowball overwintering onions @ D T Brown, I don't know anything about them though!
http://www.dtbrownseeds.co.uk/seeds-plants-gardening/62252/onions-autumn-sets-snowball

The ones in the link are for Autumn planting and will be ready for harvesting before the spring planted ones.
The only drawback is that Autumn -planted onions tend not to store as well as spring planted ones.

Spanish onions are a larger, sweeter and yellowy fleshed onion. They're fine for fresh eating and stir frys, etc, but sadly don't store quite as long as keeper types such as Stuttgart, etc. We can get them as sets here but prefer to grow them from seed due to the variety and better growth.

White onions are the sweetest yet and refer specifically to the white flesh. Unless you have perfect storage conditions, they don't last long, but they really are the best for fresh eating. Walla Wallas are such, same with Candy. These are available only as seeds or started seedlings.