Sprinkle chicken pieces with salt, pepper and paprika. Over medium heat brown chicken in half the butter 15 minutes; transfer to a shallow baking pan, arranging meat in a single layer.
Add remaining butter to frying pan and saute mushrooms and green onions until limp and lightly browned and liquid evaporates. Sprinkle flour over mushrooms and stir in chicken broth, sherry and rosemary.
Cook until thickened, then pour over chicken. (At this point if you like, you can cover and chill casserole.)
Cover and bake at 350 degrees for 1 hour. If refrigerated, 1 hour and 20 minutes.