Preheat the oven to 200 degrees celcius. Cut the slices of bread into cubes and coat lightly with cooking spray. Place on a baking tray and bake for 5 minutes or until crisp and golden. Reserve.
While the croutons are baking, place the cauliflower, potato and onion in a large pan. Add the vegetable stock and bring to the boil. Cover and gently simmer for 10 minutes or until the potato is tender.
Transfer the mixture to a food processor or blender. Process until smooth. Return to the pan and add the skim milk. Reheat and ladle into warm bowls. Scatter with the chives and the croutons.