Cook celery and onion in melted butter until soft or clear. Arrange a layer of chicken on bottom of an 8 x 8-inch buttered casserole dish. Break spaghetti into pieces and scatter evenly over meat. Blend soup, broth, salt, pepper, onion and celery. Pour mixture over all until the spaghetti is moistened. Scatter mushrooms over broth and sprinkle on cheese and paprika. Bake 1 hour at 350°. After 30 minutes, you may want to stir lightly with a fork.