Drain the beans and rinse under cold water. Heat the oil in a 5 litre (9 pint) saucepan. Add the onion, reduce the heat and cook gently for 10 minutes, stirring occasionally. Stir in the garlic and cook for 1 minute. Add the cannellini beans, chopped tomato, passata, carrot, celery and stock. Add the bay leaves and stir. Bring to the boil, then reduce the heat to medium-low and simmer, covered, for 45-60 minutes, or until the beans are tender.