Preheat the oven to 400°F.

Spray a 9x13-inch baking dish with nonstick cooking spray.

Dice the potatoes into 3/4-inch pieces and place in the baking dish. Finely mince the scallion and set aside.

Drizzle the olive oil over potatoes and season with the garlic powder, salt, and black pepper. Stir with a rubber spatula to coat potatoes in oil.

Roast the potatoes on middle rack of oven until tender but slightly underdone and just starting to brown, about 15 minutes.

Place the cooked chicken in a large mixing bowl. Season with the salt and black pepper, and add the garlic powder, paprika, minced scallion, and sour cream.

Remove potatoes from oven and transfer to the mixing bowl. Stir mixture with a rubber spatula until chicken and potatoes are thoroughly coated in sour cream.

Transfer the chicken and potato mixture to the baking dish and smooth into an even layer. Sprinkle the cheese over top.

Bake the casserole on middle rack of oven until cheese is melted and lightly golden brown, 12-15 minutes.

Check to see that casserole is done. Remove from oven or add time as needed.

Allow casserole to rest for 5 minutes before serving.