01 - Preheat your oven to 400°F (200°C). Lightly grease a 9x13-inch baking dish with butter or cooking spray.
02 - Bring a large pot of salted water to a boil. Add diced potatoes and cook for 8–10 minutes until fork-tender. Drain thoroughly and set aside.
03 - While potatoes cook, place broccoli florets in a steamer basket over boiling water. Steam for 3–4 minutes until bright green and slightly tender. Remove and set aside.
04 - In a large mixing bowl, combine cooked potatoes, steamed broccoli, cooked chicken, chopped onion, and minced garlic. Toss gently to distribute evenly.
05 - In a separate bowl, whisk together sour cream, milk, melted butter, 1 cup shredded cheddar cheese, salt, black pepper, paprika, and dried thyme until smooth and well combined.
06 - Pour the creamy sauce over the potato, chicken, and broccoli mixture. Fold gently with a spatula until all ingredients are evenly coated.
07 - Transfer the mixture to the prepared baking dish, spreading it into an even layer. Sprinkle the remaining ½ cup cheddar cheese evenly over the top.
08 - Bake uncovered for 25–30 minutes until the casserole is bubbly around the edges and the cheese on top is golden brown.
09 - Remove from oven and let the casserole rest for 5 minutes before serving to allow it to set for easier scooping.