Melt butter in medium saucepan on medium heat. Add mushrooms, celery and onions; cook and stir 3 to 5 min. or until crisp-tender.
Stir in broth. Bring to boil, stirring occasionally. Remove from heat. Add remaining ingredients; stir just until stuffing mix is moistened.
Spoon into greased 8-inch square baking dish; cover.
Bake 30 min., uncovering for the last 15 min.