Spray a frying pan with low-calorie cooking spray and set on a medium heat. Add the onions and fry for 10 minutes, until softened and turning golden brown.
Add the carrot, potatoes, and celery. Add the stock cubes to the water and stir until dissolved. Pour into the pan with the chopped tomatoes.
Bring to the boil, then lower the heat and cover with a lid. Simmer for 15-20 mins — until the veg is tender.
Using a stick blender or food processor, blend until smooth. Then season to taste. If the soup is a little thick for your liking, add a splash of extra water.