Start by trimming any large leaves off of the broccoli and removing the tough ends of lower stalks. Wash the broccoli thoroughly and then cut it into 1-inch pieces.
In a large saucepan, bring the water to a boil and add the broccoli in with the salt. Reduce the heat, cover, and simmer the broccoli for about 5 minutes, or until it is tender. Drain out the liquid and then set the cooked broccoli aside.
Separately, in another saucepan, sauté the onion, celery, and mushrooms in the margarine until tender. Then, combine the broccoli, sautéed vegetables, and water chestnuts in a bowl, set aside.
Next, combine 1/4 cup cheese, soup, and milk in a saucepan over low heat until the cheese melts, stirring constantly. Stir in the garlic powder and the pepper, and then pour over the broccoli mixture. If desired, toss lightly to more evenly coat the broccoli.
Preheat your oven to 350 degrees F and then spoon the mixture into a 2-quart casserole dish coated with cooking spray. Bake for 25 minutes and then sprinkle with the remaining 1/4 cup of cheese. Bake for an additional 5 minutes, or until the cheese melts.