Spray the bottom of a large pot, big enough to hold at least 2 litres liquid in total, 5 times, with the low cal olive oil spray. Turn on a high heat under the pot and wait until the oil bubbles, then turn down to a low heat.
Add the onion and soften it. Stirring frequently and adding small amounts of hot water from a kettle if the onions begin to stick.
Add the courgettes, stir again, and once the courgettes are soft, and before they begin to brown, add the tin of chopped tomatoes. Stir together and then add the tomato puree.
Stir the mixture together for a couple of minutes and then add the vegetable stock.
Bring the mixture to the boil and then turn down the heat to a simmer for approximately 25 minutes.
Allow the mixture to cool, then puree with a blender.
Return the pureed mixture to the pot and add salt and pepper to taste.
Serve garnished with basil or parsley