I would think that if they were well dried at the end of the year you should be able to do this just fine. From what I know, that's what the sets are that they sell in stores- just little ones that they've dried and stored.

wolverin,
You have not mentioned where you are.
planting onions is a bit complicated and it depends on the climats more than most garden vegetable. In zones 7b and higher, normally seeds are planted in fall.
In cooler and shorter summer zones it is planted in very early spring.
Also, for different climats they have different kinds of onions. For example you wouldn't want to plant the same onion seed in Florida and Michigan.
So, you have to know WHEN and WHAT to plant WHERE.

Cyrus

There's also a chance a "set" that grew all this year will bolt and go to seed if planted next year. Onions are biennials and reproduce their 2nd year or what they think may be their 2nd season. Hence bolting after temps too cold sometimes. Not to mention larger sets bolt more easily than smaller ones.

I am zone 6 and while the weather may cooperate when it is time to plant, the soil still holds the winter water and is difficult to work as early as I would like. Consequently my seeds don't have as long a growing season as I would like.
That said, I just came in from the garden and have quite a few "golf ball" sized bulbs. Given that it is still June, maybe we'll have more success with the bulbs developing than I thought we would.

I read up on this one year when I wanted to grow my own sets. Your sets need to be quite small, like the same diameter as a nickle, or they will bolt. The size of a quarter is too large. And they need to be stored over 50 degrees F for the last 30 days before you plant them next spring, otherwise, again, they bolt.

Good luck

Catherine

The odds of producing your own sets and having it work out are very low, you'd be better off starting off fresh next season and starting the onions earlier. Also make sure you have the right type for your area. You could also consider ordering bunches of starts (or buying them at a local store if they stock them), those should be at the correct size and ship/be available at the right time of year for you to get good results. Your seed grown plants should be the same size as purchased starts at the time that starts are available.

If the onions are an OP variety, you could hold over a few and they'll produce seed for you next year.

Onions are very confusing regarding day length, types and planting dates. I'd suggest searching for publications on commercial onion production in your area (or a similar climate area). Here in CA, UCDavis has extensive information on commercial crops that is very helpful to gardeners as well.