Preheat the oven to 350° F. In a small pan, saute the diced onion with a tablespoon of butter on medium heat until translucent (about 3-5 minutes). Set aside and let cool.
1 cup diced yellow onion, 1 tablespoon salted butter
In a large bowl, whisk together the sour cream, cream of chicken soup, garlic, salt, and pepper. Then add the melted butter and whisk until well-mixed.
1 cup sour cream, 1 can cream of chicken soup (10.5 oz), 2 ½ teaspoon minced garlic, 1 teaspoon fine sea salt, ¼ teaspoon ground black pepper, ½ cup salted butter
Add the thawed hash browns (do NOT add them in frozen), sautéed onion, cheddar cheese, and parmesan cheese. Stir until well-mixed.
30 oz shredded hash browns*, 2 ½ cup shredded sharp cheddar cheese**, ⅓ cup parmesan cheese
Spray the 9x13 casserole dish with cooking spray, and spread the hashbrown mix evenly throughout.
Place the corn flakes in a ziploc bag. Crush the cornflakes by hand, and then pour the cooled butter into the bag as well. Shake until all of the cornflakes are covered in butter, then sprinkle the corn flake topping evenly over the top of the hash browns.
4 cup corn flakes, ¼ cup salted butter
Place the casserole in the oven and bake for about 40-50 minutes, or until browning on the top and the cheese is bubbling. It's better to slightly overcook than undercook the casserole to make sure that the potatoes are cooked through.
Serve immediately. Enjoy!