A name invented, I think, in the 1980s, before which it was just slow-fried onions. It’s incredibly easy to make, but needs an hour or so’s cooking to get right.
2 large sweet Spanish onions 1tbs arachide or other vegetable oil 1tbs
butter 1tsp balsamic vinegar (optional) Salt and pepper
Peel and slice the onions as thinly as you can. Heat the oil in a big frying pan or wok, and slow fry the onions, turning them every 15 minutes or so (adding the butter after about 30 minutes) until they’re half the volume and very soft without being brown. Add the balsamic vinegar (optional but gives it a bite) and season with salt and pepper.
This can be done several hours in advance. When you need it, heat it up slowly and allow the balsamic flavour to seep into the onions. Ideal with sausages and mash or, my favourite, with fried calves’ liver, streaky fried bacon and mash. If you do too much, it should keep a day or two in the fridge and will heat up again.