Preheat oven to 350 degrees
Spray 13x9" casserole dish with nonstick cooking spray. Place half of the hash browns in a single layer in the bottom of the dish.
Meanwhile, in a large bowl, whisk together the eggs, milk, sour cream, salt, and pepper until smooth.
Reserve 1/2 cup of the cheese and stir the remaining cheese into the egg mixture.
Add whatever extra ingredients you want to the egg mixture.
Pour the mixture evenly over the hash browns in the casserole dish.
Place the remaining hash browns in a single layer over the top and press gently into the egg mixture.
Sprinkle with the remaining 1/2 cup of cheese. Bake until the top is golden and the egg mixture is set in the center, for 55 to 60 minutes. Let rest for 10 minutes.
If you are making this casserole 24-hours ahead, cover the top after step seven.