Preheat oven to 400 degrees F. Add olive oil to a 7-quart stock pot, and saute mushrooms, celery, carrots, onions and garlic powder for 3-4 minutes over medium heat. Add chicken broth and thyme sprig and bring to a boil, stirring occasionally. Add cream cheese, Parmesan and noodles. Stir until well blended. Sprinkle flour over top of mixture. Stir and cook for 1-2 minutes, or until mixture begins to thicken. Fold in chicken.
Pour into a 3-quart casserole dish. Sprinkle on reserved topping mixture. Bake uncovered 15 minutes. Remove from oven and let stand 10 minutes.