Heat butter in a large heavy pan (le creuset pot) over a medium heat and add the onions. Reduce the heat to low, cover the pan and cook, stirring occasionally, for 30-40 min until the onions are completely wilted and just beginning to colour.
2
Add the sugar and increase the heat. Remove the lid and cook, stirring frequently, for about 30-45min until the mixture turns dark brown and most of the liquid has evaporated.
3
Take the pan off the heat and allow to cool for a few min, then add the vinegars ( this is important, as adding vinegar to the hot pan will cause it to all steam up and evaporate away).
4
Return to the heat and cook for another 10 min or so, until the mixture becomes thick and gooey, and a spoon drawn across the bottom of the pan leaves a clear track across the base for a couple of seconds.