Spray a large saucepan with low-calorie cooking spray and set on a medium heat. Add the onions and garlic, and fry for 10 minutes. Don't stir too much, as you want the bottom to brown to give the soup its rich colour.
Add the balsamic vinegar and Henderson’s relish and stir. Pour in all of the water, then add the stock cubes and vegetable stock pot.
Continue to simmer for 25-30 minutes.
Serve the soup with a garnish of your choice!