01 - Preheat oven to 400°F. Lightly grease a 9x13-inch baking dish with cooking spray or butter.
02 - Bring salted water to a boil in a large pot. Add diced potatoes and cook for 8-10 minutes until fork-tender. Drain thoroughly.
03 - Steam broccoli florets for 3-4 minutes until bright green and slightly tender. Do not overcook.
04 - In a large mixing bowl, combine cooked potatoes, steamed broccoli, chicken, onion, and garlic.
05 - Whisk together sour cream, milk, melted butter, 1 cup cheddar cheese, salt, pepper, paprika, and dried thyme until smooth and well blended.
06 - Pour the creamy sauce mixture over the potato, chicken, and broccoli. Stir gently to coat all ingredients evenly.
07 - Transfer mixture to the prepared baking dish. Spread evenly and sprinkle remaining ½ cup cheddar cheese across the top.
08 - Bake uncovered for 25-30 minutes until the casserole is bubbly, heated through, and the cheese topping is golden brown.
09 - Allow casserole to rest for 5 minutes before serving to let the sauce set slightly for easier portioning.