If it was me. I would just plant them close 1" to 2" apart and eat them as green onions. There is know set time when you can pull a onion and just eat it. Only if you want to store them, let them go to the end.

Thanks, but I'm not interested in green onions and I know they're edible at any point. Anyone know about growing them into sets.

I googled "how to produce onion sets"

and found this among others

http://www.ehow.com/how_5698479_make-onion-sets.html

A really, really useless link, as usual with ehow. It doesn't answer the three basic questions that I asked, spacing, harvest, storage. Absolutely no help at all. I was hoping someone had some experience with actually growing them. Anyone have real-world experience they can share?

Is it worth the time?

a) the seedling grow better bigger onions than sets. you've grow seedlings at the right time this year, why not do the same next year?

b) did you even use up the seed packet? do you have more seeds for next year? worried about wasting less than a dollars worth of seeds? or you just having a hard time killing your seedlings? culling is part of vegatble gardening.

Ok, I didn't know seedlings produced better than sets. I prefer seedlings for cost, variety, and I just enjoy starting seeds.

And as for the cost, if I could produce sets from my seedlings and save them for next year and have everything else be equal then yes, I would be happy to save less than $1. That's how I roll.

Here is a much better link,
See below:

Here is a link that might be useful: Grow Your Own Onion Sets

The glitch is you have already planted your seeds and have seedlings. If you want to make your own onion sets, you don't start them early with the rest of your seedlings. You start them later. The reason being, onions bulb up (and that is after all what a set is, a very young onion plant bulbing up before you'd normally want to harvest it) in response to day length. You'd think you could just harvest some of your onions as they begin to bulb, but that's going to make storage time for your sets too long. You want to get your seedling in the ground as late as possible, but still early enough to grow a little and bulb up. Even if you do succeed, and you manage to keep your sets over winter, if the timing isn't spot-on what you will end up with are sets who won't produce much of anything next year but decent green onions. The sets folks buy in stores sometimes are those sorts of 'victims' and that's why so many of us will use only seedlings. It's a pain in the tush, for questionable results unless you just want to see if it can be done. Seed is too danged cheap, LOL.