Preheat oven: Set to 375°F (190°C). Lightly grease a 9×13-inch casserole dish.
Prepare potatoes: Toss diced potatoes with olive oil, salt, and pepper. Spread on a baking sheet and roast for 20 minutes until golden and slightly crisp. (If using cauliflower, roast for 15 minutes instead.)
Mix casserole base: In a large mixing bowl, combine shredded chicken, roasted potatoes (or cauliflower), celery, carrots, Buffalo sauce, and ranch (or blue cheese) dressing. Stir until well coated.
Assemble: Transfer mixture to the prepared casserole dish. Sprinkle evenly with shredded cheddar cheese and crumbled blue cheese if using.
Bake: Place in the oven and bake uncovered for 20–25 minutes, until cheese is melted, bubbling, and golden brown on top.
Rest & garnish: Let casserole cool for 5 minutes before serving. Garnish with sliced green onions, a drizzle of Buffalo sauce, or bacon crumbles for extra flair.