Preheat the oven to 400 degrees. Spray a 9x13 baking pan with cooking spray.
Add the ground beef and onion to a large skillet. Brown over medium heat until cooked through, about 10 minutes. Drain fat.
Add the cooked beef, hash browns, cream of chicken soup, milk, sour cream, salt and pepper, garlic powder, parsley and 1 cup of the cheese to a large bowl or just mix it all up directly in the baking dish. Stir together until evenly combined.
Spread the mixture evenly over the pan. Bake in a preheated oven for 40 minutes. Hashbrowns should be beginning to brown on the top.
Remove from the oven and sprinkle with the remaining 1 cup of cheese. Return to the oven for another 5-10 minutes, or until the cheese is melted.
Serve immediately.
Notes
This dish can be assembled up to 24 hours in advance and refrigerated. See post above for full make ahead and freezer instructions.