Oh, are we control freaks? I knew I must be a micro-manager because I love to hover and mess with them ...
Realistically, "buy more seed" and have to pay the S&H every year or two may be the most practical advice. But that's not much fun!
If onions are like Bok Choy, they may bolt to seed more readily if you plant a few at the worst time for them, instead of the optimium time. Too hot or too cold. Or stress them in other ways after they have grown a little: withhold water, or chop a lot of leaves off. Dis them verbally, or play loud rap music to them.
I can't say this from personal experience, but reading suggests that most seeds last longer if stored very dry, dark, and cool. Consistency of low humidity and temperature perhaps helps too.
My theory is that "on a shelf" in the fridge exposes them to the risk of condensation when doors open and close, so I store some seeds in a sealed jar in the fridge.
Many will say that's unecessary for most vegetable seeds, that they store some seeds in totally uncontrolled areas in open paper bags for years. I have no experience with that.
I may go too far by storing seeds in a sealed jar with silica gel dessicant in the jar. Indeed, using too much dessicant with too few seeds, sealed too long, might dry them out TOO much!
I use a humidity card be be sure the RH doesn't go below 10% for long, and the seeds are in sealed Ziplocs inside the jar, so they fluctuate even less. Each Ziploc has a paper label inside it, which buffers or evens out any humidity changes - paper holds humidity.
Now that I think about it, I'm going to add "Salvia" to the seeds I treat that way, and wrap the jar in something to keep it dark. They are also supposed to have short shelf lives, yet I WANT to keep them for years.
(Salvia might respond to Giberellic acid (GA-3) to "perk up" overage perennial flower seeds with dormancy issues. But would that help over-age onion seeds? I'm guessing "no".
>> if not sown in autumn after harvest , they almost always need a warm/cold/warm period,
I hope Jonna is right, and they have just gone into a stubburn dormancy. Maybe a few 6-packs with moist vermiculite in a bag or under plastic film, on the porch or in a fridge for a week or two, then indoors for a week or two, and repeat?
"Throw 'em out in the yard and see what happens." 
Sorry, I'm too much of a control freak. 
