I bought a hamper last Christmas and one of the items in it was a jar of Caramelised Red Onion.
I was wonderful and the ingredients were very simple, so I did try making some of my own, but only a small amount, not enough to make jars and jars of it.
The ingredients on the jar were simply, red onions, muscavado sugar, refined sugar, white wine vinegar, balsamic vinegar and salt & pepper.
So I cannot give you any amounts because I just played it by ear, I thinly sliced the onions and put them in a pan to soften, then added the sugar and heated till melted, then added the vinegars, white wine enough to cover and a large spash of balsamic. I then just brought to the boil and simmered till thick and added some salt and pepper right at the end to taste.
Probably not much help but you might like to experment with a small amount as I did.