Soak the beans overnight in plenty of water, then drain.
Coarsely chop the onion, carrot, celery, leek and tomatoes.
Finely chop the parsley.
Heat the oil in a large saucepan, then sauté the onion, carrot, celery and leek. Add the beans and thyme, stir for a few second.
Pour in the stock, add the garlic, tomatoes and thyme. Bring to the boil, then put the lid on, and simmer at low temperature for about an hour.
When the beans are tender and the soup is thick, turn off the heat. Stir in the chopped parsley.
Serve warm, sprinkled with some celery leaves.