These onions are really easy to ferment, and they only take one or two weeks. When you're done, you'll have a nice condiment to add to your salads or sandwiches. Or serve them as a side dish! For a similar type of dish, try the Instructable I made on Easy Fermented Garlic.
Step 1: Ingredients
To make the brine, dissolve salt (preferably Himalayan sea salt) in water at a ratio of about 1 tablespoon to 2 cups. If you have heated the water to aid in dissolving the salt, let it cool entirely before using it.
You can also add other things to your ferment, such as garlic, ginger, chili peppers, or herbs. In this ferment I added one head of garlic (separated into cloves).
Step 2: Chop and Stuff
Cut the top end off of your garlic cloves (if you're using them) and put them in the bottom. Thinly slice your onions and pack them into your jar. Feel free to use a tool to press the onions in there. You won't hurt them. If you want, you can layer your additional items in with the onions, but I don't find it necessary. They will be hanging out together in the brine for a week or two, and will have plenty of time to mix and mingle.
Once you have the jar mostly full, press the onions down again. Then add your brine, being sure that it covers the top of the onions completely, and that the level is an inch or so below the top of the jar.
If you have some kind of weight, you can place it on top of the onions to keep them submerged. Submerged is key in fermenting. If food is sticking above the surface, it will mold or at the very least become nasty. In my case, I cut a disk from some scrap plastic, and then weighed that down with the top to a tea kettle. Just don't use metal.
Step 3: Seal, Wait, and Then Stop Waiting
If you have an airlock, use it. They're pretty cheap and will keep the oxygen from getting into the jar. Oxygen is bad for fermentation and good for mold. If you do not have an airlock, I would advise you to seal your lid tightly and "burp" or off-gas it every day or so. Just loosen the lid enough to let out the gas, and re-seal it. You can also look online to find alternative ways of creating an airlock.
Now store your jar somewhere out of direct sunlight. I keep mine in a cupboard. Monitor it every day to be sure that the onions are staying under the surface of the brine, and also to make sure the level of the brine doesn't get too high. The salt will draw moisture out of the onions.
After a week or so, check on the taste. If you are happy with the taste, put the jar in the fridge and you're done! Otherwise, wait and check it again every couple of days. Look at that gorgeous color the brine has become!
Now take your fermented onions and add them to your salads, put them on top of your sandwiches or burgers, or just eat them raw, knowing that you're doing your body some real good.