01 - Preheat oven to 400°F. Lightly grease a 9x13-inch baking dish with cooking spray or butter.
02 - Bring salted water to boil in a large pot. Add diced potatoes and cook 8–10 minutes until fork-tender. Drain well.
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. Toss gently to distribute evenly.
05 - Whisk together sour cream, milk, melted butter, 1 cup cheddar cheese, salt, pepper, paprika, and thyme until smooth and well combined.
06 - Pour the creamy sauce over the potato-chicken-broccoli mixture. Fold gently until everything is evenly coated.
07 - Transfer mixture to prepared baking dish, spreading evenly. Sprinkle remaining ½ cup cheddar cheese across the top.
08 - Bake uncovered for 25–30 minutes until cheese is melted, bubbly, and lightly golden on top.
09 - Let casserole rest for 5 minutes before serving to allow it to set and make serving easier.