Spray a frying pan with low-calorie cooking spray and set on a medium heat. Add the onion and garlic and fry for 4-5 minutes until starting to soften.
Add the meatballs, carrot and celery and continue to fry for 4-5 minutes, until the meatballs are browned.
Add the courgettes and mushrooms, and allow to cook for a few minutes.
Add the mixed herbs, basil, tomato purée and chopped tomatoes. Stir well.
Mix the stock cube with the water and pour into the pan. Bring to the boil, then lower the heat and simmer for 15-20 minutes, until the vegetables are soft and the meatballs are cooked through.
Season with salt and pepper, to taste, and serve with your choice of accompaniment.