Preheat oven to 350 degrees F.
Spray a 9x13 pan with nonstick spray.
Break the frozen hash browns up in the bag.
In a large bowl combine the onion, 1 cup of cheese, milk, cream of chicken soup and sour cream. Stir to combine.
Mix in the hasbhrowns and stir to coat.
Pour into a 9x13 pan. Top with the remaining cheese.
Cover with foil and bake in the preheated oven for 45-50 minutes.
Remove the foil and bake for 5-10 minutes to brown the the cheese is slightly.
