Place a steaming basket in a large pot. Pour in enough water to reach just below the basket, cover and bring to a boil. Add florets and celery root cubes; steam for 15 minutes or until tender.
Meanwhile, in a saucepan over medium heat, add 2 tablespoons butter. When melted, add onions and sauté for 3 ½ to 4 minutes or until soft. Add garlic and sauté for only 1 minute. Remove from the heat and transfer onion mixture to the bowl of a food processor; set aside.
When cauliflower and celeriac are tender, transfer to the bowl of the food processor and blend until half the mixture is somewhat puréed. Add 2 tablespoons butter and process until well combined. Slowly pour in heavy cream and continue mixing until blended. Clean the sides of the bowl before adding sour cream, salt and pepper. Process once again until well mixed.
Just before serving, sprinkle on some fresh chopped parsley. Serves 6.