There is a thread that discusses onion jam recipes but I don't know that it is a specific onion jam thread. I know I put up my recipe in it.
Red Onion Jam
Diced red onion (as many as you want and will go into your pan)
Balsamic vinegar (sufficient to cover onions)
Place in saucepan together, bring to boil and then reduce to a simmer. Simmer until onions have broken down substantially, they have the glisten of jam and have significantly reduced in volume. Can't remember off-hand how long this takes - an hour perhaps? This is as simple as it gets. The big thing with this recipe is that you must have the room very well ventilated - the acidity of that volume of vinegar cooking is strong and it does linger.
If I can remember/find the thread, I will post again. Think it was early to mid last year.