Melt 4 tablespoons butter in a 4 quart saucepan. When hot, add onion, cover and simmer over low heat for about 5 minutes or until tender.
While this is cooking, peel the celery knob with a stainless steel knife, chop it roughly, then grate it in a food processor.
Add celery knob to saucepan along with potato, 1/2 cup of water, salt and pepper. Cover and simmer over low heat for 20 to 25 minutes or until celery knob is tender.
Transfer contents of saucepan to a food mill or processor and puree with butter or cream; adjust salt and pepper and garnish with parsley.