Add 4 sprays of low calorie cooking spray to the Thermomix bowl.
Add the onion and chop on Speed 6 for 30 seconds.
Cook for 4 minutes at 100° on Speed 1.
Scrape down the side of the Thermomix bowl and add the remaining veg, stock cubes, chopped tomatoes and enough water to cover the ingredients.
Cook on speed 1 at 100° for 20 minutes.
When cooked, puree on speed 10 for 30 seconds and add seasoning if necessary.