Soak the beans for 8 hours or overnight in plenty of cold water. Drain the beans, discarding the water, then place in a large saucepan. Cover with fresh water and bring to the boil. Boil rapidly for 10 minutes. Rinse the beans and drain again.
Heat the olive oil in the insert pan of a slow cooker or a large frying pan over low heat. Add the onion, carrot and celery and cook for 8 minute