Melt the butter or margarine in a large saucepan, add the onion and fry for 5 minutes until softened and just beginning to turn golden. Add the carrots and kale in batches, stirring all the time, and cook for 2 minutes.
Add the water, stock, lemon juice, potatoes and nutmeg, with salt and pepper to taste. Bring to the boil, stirring from time to time. Lower the heat, cover and simmer for 35 minutes or until all the vegetables are soft. Purée the mixture in batches in a blender or food processor, adding extra water if it is too thick.
Prepare the croûtons by cutting the bread into 1 cm (½ inch) cubes. Heat the oil in a large frying pan, add the garlic and cook over a moderate heat for 1 minute. Add the bread squares and fry, turning frequently, until evenly golden brown. Transfer the croûtons to kitchen paper with a slotted spoon to drain. Discard the garlic, then add the thinly shredded kale to the pan and fry, stirring constantly until crispy.
Taste the soup and adjust the seasoning if needed and reheat without boiling. Serve in heated soup plates with the garlic croûtons and crispy kale.