Add onion, and cook until translucent, stirring as needed.
1 small white or yellow onion
In a large bowl, stir together sour cream, cream of chicken soup, salt, pepper, dry mustard, and 2 cups of cheddar cheese.
16 ounces sour cream, 10.25 ounces cream of chicken soup, ½ teaspoon salt, ½ teaspoon pepper, ½ teaspoon dry mustard, 2 ½ cups shredded sharp cheddar cheese
Add hash browns and onions, and toss to combine.
30 ounce package frozen hash browns
Grease a 9x13-inch baking dish.
Add hash brown mixture to prepared baking dish.
Bake for 45 minutes.
Sprinkle with remaining 1/2 cup cheese.
Sprinkle the top of casserole with corn flakes and then drizzle with melted butter. Bake for an additional 10-15 minutes, or until cheese is melted and casserole is hot and bubbly.
2-3 cups cornflakes , ¼ cup butter
Video
Notes
Sour cream: I've used both lite sour cream and plain, non-fat Greek yogurt. Both work well as substitutions for regular sour cream. I don't recommend using fat-free sour cream in this recipe.
Cream of chicken soup: Either regular or 98% fat-free cream of chicken soup will work. You can also make a homemade cream of chicken soup substitute with this recipe.
Corn flake topping: You can add a cornflake topping for some crunch. Sprinkle the remaining ½ cup cheese over the top of the unbaked casserole. Add 2 cups of cornflakes over the top of the unbaked casserole. Drizzle 2 tablespoons of melted butter over the cornflakes. Bake as directed.
Make ahead: This casserole can be made through step 7, covered, and chilled for up to 24 hours prior to baking. When you're ready to bake it, set the casserole out while the oven preheats. You may find that you need to increase the cooking time depending on how cold the casserole is.