Add onion, and cook until translucent, stirring as needed.
In a large bowl, stir together sour cream, cream of chicken soup, salt, pepper, dry mustard, and 2 cups of cheddar cheese.
Add hash browns and onions, and toss to combine.
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.
Continue baking for 5-10 minutes, or until cheese is melted.
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.
Either regular or 98% fat-free cream of chicken soup will work.
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.