Preheat the oven to 350ºF (177ºC). Spray a 9x13-inch baking dish with cooking spray and set aside.
In a large mixing bowl, combine the sour cream, cream of chicken soup, milk, garlic powder, dried parsley, kosher salt, paprika, and ground black pepper. Add in the hash browns one handful at a time and stir to incorporate.
Spread half of the hash brown mixture into the bottom of the prepared casserole dish. Top with half of the chicken, half of the onion, and half of the shredded cheddar. Repeat the layers.
In a small mixing bowl, toss together the crushed potato chips and melted butter. Evenly spread the chip mixture over the casserole.
Bake uncovered in the preheated oven for 35-45 minutes or until the casserole is bubbling and the chips are lightly golden. Allow the casserole to cool for at least 10 minutes before serving.
Notes
Make ahead instructions: If you're making the casserole ahead of time, leave the potato chips off until just before you're ready to bake so they don't become soggy.Storage: Store leftovers in the fridge in an airtight container for up to 3-4 days. Note that the potato chips will not remain crispy. If you intend to eat half now and half later, I recommend leaving off the potato chip topping from half of the casserole and adding just before baking a second time.