Preheat the oven to 360 F. Pulse thawed hash brown in a food processor or chop up with a knife. Pour on to a few paper towels, roll up and gently press down to squeeze out excess water. Transfer to a deep bowl.
Add chopped onion, salt, pepper, half of the cheddar. In another bowl, mix cream of chicken soup, melted butter and sour cream until smooth. Pour liquid ingredients into a bowl with potatoes and cheese. (could add 1/2 cup precooked meat too now)
Mix everything gently with a spatula. Transfer the potato mass into a baking dish sprayed with non stick spray inside. Sprinkle the remaining cheese on top. Bake for 20 minutes.
Top with corn flakes (optional) and bake for another 7-10 minutes for added texture. Let the casserole cool slightly, then cut into portions and serve.