Preheat the oven to 350°F (175°C) and grease a 9x13-inch baking dish.

Peel and dice the Yukon Gold potatoes, then place them in a large pot covered with salted water. Boil for 10-15 minutes until tender, then drain.

In a large mixing bowl, combine sour cream, cream of chicken soup, shredded cheddar cheese, diced onions, melted butter, garlic powder, salt, and pepper.

Gently fold the drained potatoes into the sauce mixture until evenly coated.

Transfer the potato and sauce mixture into the prepared baking dish, spreading it evenly.

Sprinkle the crushed cornflakes or potato chips over the top of the casserole.

Bake for 30-35 minutes until bubbly and golden brown.

Let cool for 5-10 minutes before serving.