Step 1: Oven and Pan Preparation: Preheat your oven to 350°F (175°C). Grease a 9×13 inch baking dish with non-stick cooking spray or a little bit of butter to ensure the potatoes don’t stick to the sides.
Step 2: Mixing the Creamy Base: In a large mixing bowl, combine the melted butter, cream of chicken soup, sour cream, diced onions, salt, and black pepper. Stir everything together until the mixture is smooth and well-incorporated.
Step 3: Folding in Potatoes and Cheese: Add the thawed shredded hash browns and the shredded cheddar cheese to the bowl. Use a large spatula to gently fold the potatoes into the creamy mixture until every shred is evenly coated with sauce and cheese.
Step 4: Assembling the Casserole: Spread the potato mixture evenly into the prepared baking dish. Smooth the top with your spatula so it cooks evenly. In a small separate bowl, toss the crushed cornflakes with the remaining 2 tablespoons of melted butter and sprinkle them over the top of the potatoes.
Step 5: Baking to Golden Perfection: Place the dish in the center rack of the oven. Bake for 45 to 50 minutes, or until the edges are bubbling and the cornflake topping is a beautiful golden brown color. Remove from the oven and let it sit for 5 minutes before serving.