Steam cauliflower for 5 minutes until florets are just tender. Set aside but save cooking water. Parboil potatoes for 10 minute or until tender. Set aside but save cooking water.
In non-stick skillet, melt butter, add garlic and onion and saute until soft. Combine reserved cauliflower, potatoes, garlic/onion mixture and parsley and puree in blender or food processor in batches, adding the reserved liquid as needed.
Transfer the puree to a large saucepan and add additional cooking liquid to achieve a consistency somewhat thicker than ultimately desired. Stir in soy sauce, cardamom, salt, pepper, and cayenne. Add milk, stirring soup to blend. Reheat the soup; do not boil. Garnish with scallions and serve with croutons.
