As those are cooking, it’s time to start cooking the onions and mushrooms. We want to basically caramelise the onions, so put them in first and add some balsamic vinegar. For some reason the balsamic vinegar we were using that day came in spray form. It’s not essential that your balsamic vinegar comes in spray form, although it is quite cool.
Then add a bit of honey just to make it a bit sweeter.