Heat a large non-stick frying pan over a medium heat and lightly spray with oil. Add the sliced onion with a pinch of salt and cook for around 15-18 minutes, stirring regularly, until very soft, golden and lightly caramelised. If the onions start to catch, add a small splash of water and continue cooking. Add the balsamic vinegar and cook for another 1 minute, stirring until the onions are glossy. Remove from the pan and set aside.