When I had a Conserves company, we used to make an onion "confiture" ( French for jam - just sounded better in French!).
Sliced about 10 kgs of onions at a time, and into a HUGE stainless steel pot, with a bit of vinegar, some sugar ( not too much), some green peppercorns... cooked for about 3 hours and stirred frequently. OMG with a BBQ.
I make onion jam every year at Christmas for gifts, though usually these days, I make Bacon Jam which is pretty much the same thing. But -- it has bacon! Carmelized onions and bacon!