Chop onions in half, lengthwise. Cut off the top and tail and peel the skins. Slice onions into thin half moon shapes.
Heat about 6 tablespoons oil in a large pot and fry the onions, covered over a medium low heat until the onions are super soft. About 40 minutes.
Remove the lid and add balsamic and cook the onions, uncovered on a medium heat until the onions are well browned and caramelised. Another 15-20 minutes.