Onion sets

Senshyu are considered a mild onion ... perhaps try Snowball for winter growing?

For summer, there are plenty of sweet varieties (Spanish white type or maybe a red onion?)  Many however, have to be grown from seed rather than from set.  I'm sure others can recommend one for you (I prefer strong flavoured onions so not a lot of help, sorry)

I grew Ellista this year from plants bought from DT Brown. They seem to be sweet & mild.