Allow the shredded hash browns to thaw in the refrigerator overnight, or for several hours on the countertop.
Preheat the oven to 350ºF (177ºC) and grease a deep 9X13" (23x33 cm) baking dish with cooking spray.
In a large bowl, combine the sour cream, gluten-free cream of chicken soup, ½ cup of melted butter, salt, onion powder, and pepper.
Fold everything together with a silicone spatula until smooth.
Add the thawed hash brown potatoes and stir together until all of the potatoes are coated with the sauce.
Finally, sprinkle in the shredded cheddar cheese and fold again.
Pour the potato and soup mixture into the greased casserole dish, spreading it into an even layer.
Crush the gluten-free corn flakes by placing them in a plastic storage bag and pressing them firmly (or smashing them) with a rolling pin.
Melt the butter in a medium-sized bowl in the microwave using 30-second intervals.
Once the butter is melted, add the crushed cereal to the melted butter and stir to coat.
Sprinkle the crushed corn flake and butter mixture on top of the casserole, ensuring the topping is evenly distributed.
Place the dish in the oven and bake uncovered for 40-50 minutes, or until hot and bubbly.
Remove from the oven and allow to cool for 5-10 minutes before serving.
Notes
Storing: Transfer leftovers to an airtight container and keep in the refrigerator for up to 3 days. Reheat in the microwave or a warm oven. Freezing: Assemble the casserole (leaving off the topping for the best results) and freeze for up to 3 months. Thaw before baking, then sprinkle on the prepared topping (if not prepared when freezing) and bake as directed. (Note: To avoid temperature shock breaking your dish, add the cold dish to the cold oven, then proceed to preheat).