Soak beans in cold water overnight or at least 3 to 4 hours. Drain and place into a saucepan with water to cover. Slowly bring to a boil. Add half the onion, the carrot, and the celery, and cook until tender. In the meantime, thinly slice the remaining half onion. When beans are soft, drain them and discard the vegetables. Mix oil, vinegar, and garlic in a separate bowl, then set aside. While beans are still hot, stir in sliced onion and herbs. Add the oil and vinegar dressing, then toss lightly. Serve immediately, while hot.