Heat the oven: Set to 375°F (190°C). Grease a 9x13-inch baking dish.
Prep the base: If using potatoes, parboil until just tender, then drain well.
If using pasta or cauliflower, cook until al dente/tender-crisp, then drain. Excess moisture will water down the casserole, so let it steam off.
Sauté the veggies: In a skillet, melt butter over medium heat. Add celery, bell pepper, and garlic. Cook 3–4 minutes until slightly softened.
Season lightly with salt and pepper.
Make the sauce: In a large bowl, whisk cream cheese, Greek yogurt (or sour cream), buffalo sauce, and ranch until smooth. Stir in onion powder and smoked paprika. Taste and adjust heat or salt.
Combine the filling: Add chicken, sautéed veggies, green onions, and half the shredded cheese to the sauce.
Fold in your base (potatoes, pasta, or cauliflower) until everything is coated.
Assemble: Spread the mixture in the prepared baking dish. Top with the remaining shredded cheese. Sprinkle blue cheese crumbles if using.
Bake: Bake uncovered for 20–25 minutes, until hot and bubbling.
If you like a browned top, broil for 1–2 minutes at the end. Let it rest 5–10 minutes so it sets before serving.
Finish and serve: Garnish with parsley and an extra drizzle of buffalo or ranch if you like. Serve with a crisp salad or simple steamed veggies.