Cut broccoli into 1 inch pieces and cook 5 to 8 minutes in salted water. Drain well. Sauté onion, celery and mushrooms in butter; drain. Stir together mushroom mixture, broccoli and water chestnuts; set aside.
Combine Velveeta cheese and soup; cook over low heat, stirring until cheese melts. Add garlic salt and pepper. Pour over broccoli mixture and spoon into a greased 2 quart casserole.
Bake at 350 for 25 minutes; sprinkle with grated cheese and bake an additional five minutes.
Note: This is really good and is worth the number of pans, however, I have also successfully made this using only one pan. I use an oven proof/stove top casserole and sauté the vegetables. The broccoli is placed atop and then steamed for about 5 minutes. about 3 tablespoons of water may be added, if desired. The remaining ingredients are added and the container covered for another 5-8 minutes to allow the cheese to soften. Remove cover and stir gently to blend cheese and soup with other ingredients. Place in preheated oven to bake as directed. I actually think this method of preparation is better as the broccoli does not get overcooked and the sauce seems to be creamier.
Prepare casseroles and freeze before baking. Unthaw and bake as directed.