Spray a 9x13-inch baking dish with cooking spray. Place hash browns in bottom of pan.
Brown sausage and onion in skillet. Drain fat.
Whisk together eggs, milk, salt and pepper. Pour over hash browns.
Spoon sausage and onion mixture over eggs, top with cheese.
Cover with foil and bake at 350 degrees for one hour. Uncover and bake an additional 15 minutes until eggs are set.
Recipe can be made the day ahead, covered and refrigerated.