Place lentils, bay leaf and oil in a medium saucepan, with water to cover by 2 inches. Cook until tender but still holding their shape (about 35 minutes). Add more water if the level ever drops below the level of the lentils.
Warm oil in a soup pot. Add garlic, onion and celery. Sprinkle with a pinch of salt and cook until onions are tender. Add potatoes, carrots, and herbs. Cook, stirring, for 2-3 minutes, to bring out the aroma of the herbs. Add tomatoes with juice. Refill can with water and add to soup. Cover and simmer for 10 minutes.
Add lentils, with their cooking liquid and the bay leaf. Add pepper and salt. Cover and simmer until all vegetables are cooked, about 20 minutes. Taste and adjust seasonings if needed.