Bring a large pot of water to a boil. Cook cauliflower for 8 to 10 minutes; drain and cool.
In a large mixing bowl, gently stir together cooled cauliflower, 1/2 of the crumbled bacon, 3 tablespoons chives, sour cream, mayonnaise, 1 cup of cheese, and sliced mushrooms. Season with salt and pepper.
Pour into a 9 x13-inch baking dish and sprinkle with remaining 1 cup of cheese and bacon crumbles.
Bake for 15-20 minutes, until cheese is melted. Top with remaining 3 tablespoons chives and serve.