Step 1
Preheat oven to 425 degrees F (220 degrees C). Grease a 9×13-inch baking dish.

Step 2
Place turkey bacon in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain the bacon slices on paper towels and crumble.

Step 3
Place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Bring water to a boil. Add cauliflower, cover, and steam until tender, 15 to 20 minutes.

Step 4
Combine sour cream, mayonnaise, garlic, salt, and black pepper in a large bowl; add cauliflower, 1/2 of the bacon, 1 cup Colby-Monterey Jack cheese, mushrooms, and 3 tablespoons chives and mix well. Transfer mixture to the prepared baking dish and top with remaining bacon and cheese.

Step 5
Bake in the preheated oven until bubbling and cheese is melted, about 20 minutes. Sprinkle remaining chives over casserole.