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Place the bag of frozen hashbrowns in the refrigerator to thaw. You'll want to do this a few hours before you plan to make the casserole. Your other option, if you need them to thaw faster, is to microwave them in a microwave safe bowl. To ensure there isn't moisture present after microwaving, place the hashbrowns in a paper towel and squeeze out any moisture.
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Preheat oven to 350 degrees.
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Spray a baking dish with cooking spray. If freezing overnight, spray an aluminum pan or line your baking dish with aluminum foil before spraying.
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Combine the thawed hashbrowns, melted butter, cream of chicken soup, chopped onion, and sour cream in a large bowl. Add the 2 cups of shredded cheddar cheese and mix until combined.
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Spoon the hashbrown mixture into your baking dish/aluminum pan and spread evenly. If you are serving immediately after cooking, place the 1/4 cup of extra cheddar cheese on top. If you are freezing for later, i'd skip this step and put it on when you reheat it.
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Bake the casserole for 45 minutes or until cooked through and all of the cheese is melted.
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Serve immediately, or if freezing, set aside to completely cool. Once cooled, wrap up in aluminum foil or place a lid on an aluminum pan and freeze until ready to use.
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To reheat frozen casserole: If in an aluminum pan, preheat oven to 400 degrees and cook for about 30 minutes. I stir mine every 10 minutes once it's able to be stirred. Once it's almost done, you can spread it out evenly again and top with cheese to be melted in oven. If you used something other than aluminum to freeze your casserole, you can place on a microwave safe plate or container and microwave for approximately 10-15 minutes, stirring to ensure all of it is cooked evenly. You can then top with cheese and put back in microwave or finish in the oven using an oven safe container.