I like to make pickled onions, a traditional British thing (especially around Christmas time). Compared to all this pressure canning stuff, it's really easy.
Buy some small onions (like, mouthful-sized)
Leave them overnight in a colander filled with pickling salt.
The next day, combine the following:
- Chili peppers of some sort; habaneros will work fine.
- Pickling spices (cloves, cinnamon, etc, but you can get them ready mixed)
- Vinegar - traditionally malt but cider works fine too.
Bring all that shit to the boil, then let it cool completely.
Take some Ball jars, cleaned with boiling water so they're not festering seats of infection.
Put onions in jars with vinegar and spices, filling the jar to the top.
Seal and leave in the back of the cupboard for two weeks or more.
Enjoy either whole or sliced on good strong cheddar in a sandwich. Once the jar is opened I tend to keep it in the fridge, but I'm not sure that's actually necessary.
You're preserving the onions in vinegar, so generally speaking your chances of icky germs are pretty low. The worst that'll happen is they'll go soft, like the store-bought pickled onions you can sometimes find in the 'ethnic' aisles of US supermarkets (because they import the cheapest variety they can find, leave it on a boat for weeks, then sell it at a premium to poor homesick expats). Proper pickled onions are supposed to be crunchy, and also to blow your head off.