In a medium saucepot, heat oil over medium-high heat until hot. Add onion and celery, and cook, stirring occasionally, until browned and tender, 8 to 10 minutes. Stir in apples and broth, and bring to a boil. Reduce heat to medium-high and simmer until apples are very tender, about 10 minutes.
Transfer apple mixture to a blender. Add Brie and salt, and purée.
Return soup to the saucepot and heat over medium heat until hot. Serve garnished with thyme.
Per Serving: Serving size: about 1 cup, 160 calories (60 from fat), 7g total fat, 3g saturated fat, 15mg cholesterol, 310mg sodium, 22g carbohydrates, (3 g dietary fiber, 16g sugar), 4g protein.