Preheat oven to 400°F (200°C).

Toss diced potatoes with olive oil, salt, and black pepper. Spread in a single layer on a baking sheet.

Roast potatoes for 20–25 minutes, stirring halfway, until golden and tender.

While potatoes roast, steam broccoli for 4–5 minutes until tender-crisp. Drain well.

In a skillet, melt butter over medium heat and warm the cooked chicken until heated through.

In a bowl, whisk together sour cream, milk, garlic powder, onion powder, and 1 cup cheddar cheese.

Add roasted potatoes, chicken, and broccoli to a greased casserole dish.

Pour sauce over the mixture and gently stir to coat.

Top with remaining cheddar cheese and Parmesan.

Reduce oven temperature to 350°F (175°C) and bake for 20–25 minutes, until bubbly and golden.

Rest for 5 minutes, garnish with parsley, and serve warm.