It started last fall when I needed to use 20lb of onions I could not store. I've since made a second batch and work and it turned out far better than the first. I'm sorry I don't have exact measurements yet but if you know how to can you should be able to dial it in.
Onions: Minced or Julienned, 10 cups
Sugar: White regular sugar 6+2-3 cups
Apple juice: or cider, whatever 6 cups
Apple cider Vinegar: 2-3 cups
You need 2 pans for this. I use stainless steel for jamming to prevent flavor transfer. You start by cooking the onions in a dry pan over medium heat to caramelize them. This can take 15 min or more and you have to stir frequently to prevent scorching. You know you are done when the onions are translucent and there is a thick layer of caramelized onion stuck to the bottom of the pan.
In the second pan start making your caramel. A good video of the process is here
just use a regular pan instead of a pan for flan. deglaze the caramel with the apple juice. It will instantly crystallize, this is ok, just keep stirring. When all the sugar is dissolved pour the caramel sauce over the onions and return them to the heat. Bring to a boil and cook as you would to make a regular jam. When it is close to being done add the vinegar and enough additional sugar to give it the jam like consistency you want. When you reach 220 jar it up and water bath can for storage.
It should be noted this recipe uses NO oil. This is done intentionally to make it safe to can. It should also be noted that it's not going to be as thick as regular jam due to not having allot of pectin.
My idea for making this was to have caramelized onions available to use whenever I wanted. Just spread it on a burger/whatever. I have found it to be phenomenal on pork.