Spray a 9"x13" pan with nonstick cooking spray.
Preheat the oven to 350 degrees Fahrenheit.
Pour frozen hashbrowns into a large bowl and season with salt and pepper, then toss to distribute the seasonings evenly.
In another large bowl, stir together diced onion, cream of chicken soup, sour cream, and melted butter. Add the grated cheddar and stir.
Pour the sauce mixture over the seasoned potatoes and stir to coat evenly.
Pour the potato mixture into the prepared pan and spread evenly. Dish will be full.
Bake, uncovered, for about one hour, if using frozen potatoes, or just until it's fully heated through and nice and warm.
For thawed potatoes, it should be done in around 30 minutes.
Serve, topped with additional shredded cheddar and diced chives or green onions or crushed cornflakes.