Preheat oven to 370 degrees. Spray a 11×7 (you can also use 13×9) baking dish with non-stick cooking spray.
Fry bacon in a large skillet until crispy and crumble. Set aside.
Steam cauliflower until tender, about 15 to 20 minutes.
Combine your sour cream, mayonnaise, ranch seasoning, black pepper in a large bowl. Add the steamed cauliflower florets, ½ of the bacon, 1 cup sharp cheddar cheese and 3 tablespoons chives; mix well. Transfer mixture to the prepared baking dish and top with your colby & monterey jack cheese and the other half of the bacon.
Cover dish with foil and bake for 20 minutes. Remove foil and bake another 5-10 minutes or just until cheese is bubbly and beginning to brown. Garnish casserole with remaining chives.