Preheat oven to 425 degrees F (220 degrees C). Grease a 9x13-inch baking dish.

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.

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.

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.

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