Cut off and discard the tough ends of the broccoli stalks. Break the florets into small clusters, and steam for about 10 minutes until al dente. Refresh in cold water. Drain.
Mix together the mushrooms, sour cream, mayonnaise, sugar, pepper, onion and garlic in a small bowl.
In a large salad bowl, combine the broccoli and water chestnuts. Add the creamed mixture, and toss gently. Cover with cling film, and refrigerate for at least 4 hours before serving.