Combine beans and water and bring to a boil. Skim off and discard any foam that rises to the top. Add the celery, kombu, and bay leaf. Reduce the heat to low and simmer, covered, for 30 minutes.

In a sauté pan over medium heat, warm the oil. Add the onions, garlic, rosemary, and a pinch of salt. With a wooden spoon, turn the vegetables several times to coat them with oil. Cover, reduce the heat to low, and cook gently for 30 minutes, or until the onions and garlic are meltingly tender.

Remove the celery, kombu, and bay leaf from the beans and discard. Scoop out 1 cup of the bean water. Add the onion mixture to the beans, then deglaze the sauté pan with the bean water. Return this to the soup and continue simmering until the beans are tender and crush easily when pressed against the side of the pot with the back of a spoon, 45 minutes to an hour.

Pass the soup through the fine holes of a food mill, purée with a handheld immersion blender, or purée in a food processor.

Add additional water to thin the soup to desired consistency. Add the lemon juice, salt, and pepper to taste. Serve piping hot.

Garnish with chopped parsley, and if desired, crispy pieces of smoked bacon. Serve with croutons, crostini, or slices of baguettes.