FABULOUS MUSHROOM AND BROCCOLI CASSEROLE !

1 lb. of mixed mushrooms, cremini, shitake, baby portabellas, etc., stemmed and quartered 1 stick butter, plus 1 T. melted 1 large Vidalia onion, minced 4-6 celery ribs, finely diced 3 T. flour 1 c. chicken stock or broth ¼…