Trim off large leaves of broccoli; remove tough ends of lower stalks. Wash thoroughly; cut into 1" pieces. Bring water to a boil in large saucepan; add broccoli and salt. Cover, reduce heat and simmer 5 minutes, or until tender. Drain liquid, and set broccoli aside. Saute onion, celery and mushrooms in butter in saucepan until tender. Combine 1/4 cup cheese, soup and milk in saucepan; cook over low heat, stirring constantly, until cheese melts. Combine broccoli, sauteed vegetables and water chestnuts in a large bowl; set aside. Stir in garlic powder and pepper. Pour over broccoli mixture, tossing lightly to coat. Spoon mixture into a 2 quart casserole coated with cooking spray. Bake at 350 degrees for 25 minutes. Sprinkle with cheese and bake 5 minutes.