Preheat oven to 425 degrees.
In a large pot, boil water and cook cauliflower for 8-10 minutes. Drain and let cool.
In a large bowl, combine sour cream, mayonnaise, 1/2 of crumbled bacon, 3 T. chives, 1 c. cheese, cauliflower and mushrooms, if using. Mix well.
Place mixture in baking dish and cover with remaining 1 cup of cheese and bacon.
Bake for 15-20 minutes until cheese is melted. Top with remaining chives and serve.