Keep it simple, keep it light, use good quality bought stock, or even better your own, if you have it (i never do).
Examples - fry onions, carrots (celery, if you like it, I hate it), diced finely. Add tons of chopped tomatoes, 1 pint of good stock, bouquet garnet and handful of basil. bring to the boil and simmer for 45 mins, add pepper as required.
Serve immediately with crusty bread, if you have it, a swirl of yoghurt on top.
Add handful of small soup pasta for a bulkier meal about 15 mins before it finishes, or a handful of red split lentils at the beginning.
If you are not on a diet, add cream and blend.