To get the most sweetness out of the onions with little added sugar, the key is high heat, regular stirring and patience. It is worth making this in a larger batch, rather than single meal serve, so not to rush the cooking of the onions. I find using a wok, if you have one, ensures even cooking without the possibility of the onions stewing, rather than caramelising.
4large brown onions, about 700g
2tbspbalsamic or red wine vinegar
1tbspfresh rosemary leaves or 1 tsp dried
Peel the onions, and thinly slice. This can be done in a food processor with a slicing attachment.
Place the sliced onions into a large bowl and toss in the salt. Set aside to soften for 5 minutes. The salt will help to draw moisture out of the onions while cooking to caramelise faster.
Heat a wok or large heavy-based frying pan over a high heat. Add the sliced onions and cook for 15 minutes, stirring regularly, until the onions have reduced in bulk and evenly golden brown.
Add the remaining ingredients and continue to cook for another 10-15 minutes, until sticky and darker in colour. Cool a little then scoop into a clean jar. Store in the fridge and use within 2 weeks.