Preheat oven to 375°F (190°C). Grease a 9×13-inch baking dish.
Line the bottom of the baking dish with the hashbrowns. You can put them in whole or break them up to cover more surface area. They can be frozen or thawed.
In a large bowl, combine eggs, milk, salt, and pepper. Mix until well combined. Pour the mixture into the prepared baking dish. Top with cheese.
Bake in the preheated oven for 30 minutes, or until the casserole is golden brown and bubbly.
Let the casserole cool slightly before serving. Top with sour cream, tomatoes, and avocado.