Finely chop the onions. In a large non-stick pan fry the onions gently for about 3 minutes.

Add the balsamic vinegar and turn up the heat. Then add the sugar, salt and water. Bring up to a boil for 5 minutes until the onions are soft, then reduce the heat.

Reduce down the onions on a medium to low heat, stirring so they don't stick. As the balsamic reduces you should see it thicken and the onions darken. After about 25 minutes they should be well cooked, you shouldn't be able to tell the difference between the white and red onion.

You want the onion to be very soft, sweet and sticky. Chill and put in a jar and enjoy! It should last about a week in the fridge.