Fill a bowl with cold water and add the lemon juice. Peel the Jerusalem artichokes using a potato peeler and immediately place them into the bowl of lemon water.
Meanwhile, heat the oil in a large pan. Add the bacon and fry for 4-5 minutes or until golden-brown and crisp, occasionally stirring with a wooden spoon. Remove one tablespoon of the bacon lardons and set aside for garnish.
Add the butter to the pan the bacon was cooked in. When the butter is foaming, add the onions and stir. Reduce the heat and sweat for 7-8 minutes.
Thinly slice the artichokes and add them to the pan.
Add the water to the pan. Season, to taste, with salt and freshly ground black pepper. Bring the mixture back to the boil, then simmer for 35 minutes or until the artichokes are tender.
Transfer the mixture to a food processor and blend until smooth.
To serve, divide the soup equally among six bowls. Garnish with chopped parsley, the reserved tablespoon of bacon and olive oil croutons.