Prepare the Baking Dish
Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish with cooking spray or butter.
Layer the Ingredients
Spread the hash browns evenly in the prepared baking dish.
Sprinkle the shredded cheese, onions, bell peppers, and optional cooked sausage over the hash browns.
Make the Egg Mixture
In a mixing bowl, whisk together the eggs, milk, salt, and pepper until well combined.
Pour and Bake
Pour the egg mixture evenly over the layered ingredients in the baking dish.
Bake uncovered for 40-50 minutes, or until the eggs are set and the top is golden brown.
Rest and Serve
Allow the casserole to cool for 5 minutes before slicing. Garnish with fresh parsley if desired, and serve warm.