Wash the beans and put them in a saucepan along with the water. Cover loosely, bring to a boil, and simmer for about 1 1/2 hours.
Meantime, chop the onion and saute in the oil with garlic until soft. Crush the garlic. Chop the celery, including the leaves. Dice potato and carrot. Add celery, potato, and carrot to onion and cook for several minutes, stirring.
Add the vegetables to the beans, along with the seasonings. Bring the soup to a boil, then lower the heat to a simmer until the beans and vegetables are done, about another hour. Mash about half of the soup. This produces a very thick soup. Thin with additional water to desired consistency.
Juice the lemon and stir in juice just before serving. If desired, garnish with slices from additional lemon.
