Heat a non-stick frying pan over a medium heat and add the sliced onion with a pinch of salt. Cook for around 20 minutes, stirring regularly. If the onions start to catch before they’ve softened, add a small splash of water and continue cooking. Once soft and golden, stir through the balsamic vinegar, if using, and cook for another 1-2 minutes until slightly sticky. You can cook the onions a little quicker if preferred, but giving them longer will make them softer and more jammy.