Melt the butter in a large saucepan over medium heat; cook the onions and celery until the onions are translucent, about 5 minutes. Stir in the flour, and cook for 3 minutes, stirring constantly. Gradually whisk in the chicken stock. Simmer until the celery is tender, about 20 minutes, stirring occasionally. Stir in the Brie cheese, and allow to melt for about 5 minutes.