So. Cal is a very specific area for onions. Many of them are seeded in the fall and harvested before it gets too hot in May/June. I grow some like this in NW Oregon, but you're area is much more suited for it.
If you want a yellow onion, try HI-keeper or Top-keeper from territorial. If you want a red, try Desert sunrise. If you want a sweet, try Candy (day neutral).

Just a couple of suggestions. There are plenty more short day onions out there, but I haven't tried them. Hopefully someone from near you will chime in.

-Mark

I would look at the sweet onions from Georgia (Granex) and Texas (Grano). These and other SD onions are seeded in September and harvested in late spring.

Here is a link that might be useful: CA onion varieties

Thanks for the replies. I have seeds for a spicy red onion already called Red Bottle. I just want a good sweet onion to go with it. I like cooking and adding lots of different flavors for complex flavors. So that is why I want a sweet and a spicy onion. I will take look at those recommended varieties.

I am not sure whether growing conditions in Southern Cal are sufficiently similar but D. Palmer (Yuma Arizona) developed many of the short day varieties grown in Georgia. also quite a few of those grown in the Pacific Northwest. The granex types do quite well as a sweet onion. The red Pinot Red has done exceptionally well for me. Whites ahve grown well but thus far have been small.
http://www.dpseeds.com/onions_seeds
{{gwi:101468}}

I have a similar case and question:

I am in PNW, Seattle, WA area. I have started some onion seeds in August. The seeds were labeled as scallion, and showed red bulbs in the picture on package.
So, actually I bought it with the intention to be used as scallions.
Right now the seedlings are thin and just trying to stand up. I have transplanted them from original flat to several pots. I dont have space fro them till the tomatoes are finished.

Now the question is: How/will they will overwinter here ? And when they will have some bulb ?

I am doing this just as an experiment. If nothing else, we can just harvest them as scallions.

I think that if your seeds were labeled as scallions, they will be just that.
Scallions are selected by breeders for their tendency not to bulb.

As far as overwintering, you should have no problem unless it gets super cold this winter. Most onions can take temps down to 15F. I'd think you'll have till April to eat them before they go to seed.

-Mark

Thanks Mark
I suspected also, that what I got might not bulb. But that is ok .
My main concern was about overwintering. We can use it as scallions at various stage. The timing is good too, when I am finished with them , will be tomato planting time.