Spray the inner pot of the Instant Pot with low-calorie cooking spray, and set to 'Sauté'. Add the onions and garlic, and fry. Do not stir too frequently — you want the bottom to brown to give the soup a nice rich colour. This will take around 10 minutes in total.
Add the balsamic vinegar and Henderson’s relish, and stir. Pour in all of the water, and add the stock cubes and vegetable stock pot.
Put the lid on, ensure the pressure valve is closed and press the "Manual" button. Change the time to 5 minutes. Bear in mind that the Instant Pot may take a little while to come up to pressure as there's quite a lot of liquid in there!
Once the Instant Pot has beeped to let you know it's finished, “Quick Release” the pressure (careful not to get in the way of the steam as it releases!). Then serve the soup with a garnish of your choice. Voila!