Soak the beans overnight in plenty of water in a large covered soup pot. Or cover with water, bring to a boil, then let stand off the heat for an hour for a shortcut version of presoaking.
In either case, drain the beans after soaking, and rinse. Place in a soup pot with fresh water in a ratio of approximately 3 parts water to 1 part beans. Bring to a simmer, then cover and simmer steadily for 1 hour.
Add the onion, carrots, celery, garlic, parsley, bay leaves, seasoning, nutmeg, and optional wine. Simmer for another 1 to 1½ hours, or until the beans are soft.
Scoop out about 1½ cups of the beans with a slotted spoon, avoiding as much as possible scooping out the other vegetables. Set aside.
Discard the bay leaves and transfer the solid ingredients, in batches, to a food processor or blender. Use about ¼ cup cooking liquid per batch. Process until smoothly pureed, then return the puree to the soup pot along with the reserved beans. Or insert an immersion blender into the pot and process until smoothly pureed. Season with salt and pepper and return to low heat for 15 minutes.
Just before serving, heat the oil in a small skillet. Add the sliced onion and sauté over medium heat until golden brown.
Garnish each serving with some of the sautéed onion, 2 lemon slices, and some chopped parsley. This soup keeps very well for several days, and the flavor improves as it stands.