Caramelised onions go well in pretty much any kind of veggie burger (and carnivores won’t say no either).
Makes enough for four burgers.
Finely slice the onion. In a hot fry pan add the oil, onion and a pinch of salt and sauté for 2 minutes and then turn the heat down to medium. Leave to cook until the onions turn brown; about 5 minutes.
Add the sugar and mix through until the onions are glossy and the sugar is dissolved. Add the vinegar and cook for another minute until the vinegar is absorbed. When the onions are soft stir through the mustard and then serve.