Peel and slice the celery roots as for Celery Root Nature.
Place in a microwave-safe dish with the milk, salt and sugar.
Microwave at HIGH 8 minutes.
Drain, reserving the milk.
Puree in a food processor, a blender, or mash by hand.
Add the parsley, butter, salt and pepper to taste, and a little milk if necessary.
Place in a vegetable dish, cover.
Reheat 2 minutes at HIGH.