1. In a pan, on the lowest heat, add oil, onion, salt and pepper.
2. Cook for around one hour, stirring frequently until deeply golden and sticky.
3. The last minute of cooking add garlic and balsamic vinegar. Set aside to cool for five minutes.
4. In a large bowl, combine the rest of the ingredients. Add three-quarters of the onion mixture (reserving the last quarter to eat with your burger) and mix thoroughly to combine all the ingredients.