In a heavy saucepan over medium-low heat, melt butter. Add onions and celery and sauté 5 to 10 minutes or until softened. Add flour and cook 1 minute.
Gradually add liquids, stirring constantly until slightly thickened. (Soup can be made in advance to this point and stored in refrigerator until serving time.)
Over low heat, add cubes of Brie cheese. Use a heat-proof rubber spatula and stir constantly until all of cheese has completely melted. Add salt and pepper as desired.
Garnish with Parmesan cheese and thinly sliced green onions.
SERVES: 4 - 6