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.

Pour the soup into a blender, filling the pitcher no more than halfway full. Hold down the lid of the blender with a folded kitchen towel, and carefully start the blender, using a few quick pulses to get the soup moving before leaving it on to puree. Puree in batches until smooth, and pour into a clean pot. Heat the soup to almost simmering over medium-low heat. Pour in the cream. To serve, garnish with red and green pepper strips.