Roughly chop the onion, carrot, garlic & celery.
Heat the oil in a saucepan and gently sauté the chopped vegetables until they're softened, stirring a couple of times. This will take about 10 minutes.
Add the tinned tomatoes and sugar, if used.(If you’re using fresh tomatoes, wash them, then chop them and add to the vegetables.)
Add the stock, bring to the boil, then simmer for 20 minutes under cover.
Add the basil leaves and, using a liquidizer or hand-held blender, process the soup until smooth and silky.
Drain the beans and add to the soup. Taste and adjust the salt, pepper and sugar, if needed.
Warm up the soup, if needed.
Serve with freshly ground pepper, garnished with basil leaves.