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.