In a large skillet, sauté mushrooms and celery in 3 T. butter until tender. 

     Combine cornstarch and milk until smooth; stir into mushroom mixture.  Bring to a boil over medium heat, stirring constantly.  Cook for 1 minute or until thickened.

     Stir in the turkey, noodles, broth, salt, pepper, and thyme.  Pour into a greased 2 qt. baking dish.  Melt 2 T. butter; toss with breadcrumbs.  Sprinkle over casserole. 

     Bake, uncovered at 375 degrees for 20-25 minutes or until heated through.