Preheat oven to 350°F and lightly grease a 9x13 inch baking dish.
In a large bowl, combine hash browns, cooked sausage, diced onions, bell peppers, and 2 cups of cheese. Mix gently and spread evenly in the prepared dish.
In a separate bowl, whisk together eggs, milk, salt, and black pepper until well combined.
Pour the egg mixture over the hashbrown mixture and top with the remaining 1 cup of cheese.
Bake uncovered for 55 minutes or until casserole is golden and bubbly around the edges.
Notes
This casserole is easy to prepare ahead and perfect for feeding a crowd.