Preheat oven to 350°F (175°C).
In a large bowl, whisk together eggs, whole milk, salt, black pepper, garlic powder, and onion powder until well combined.
Spray a 9×13-inch baking dish with non-stick cooking spray.
Spread the thawed hash browns evenly in the bottom of the prepared baking dish.
Layer the cooked and crumbled breakfast sausage evenly over the hash browns.
Scatter the diced bell pepper and diced onion evenly over the sausage layer.
Sprinkle the shredded cheddar cheese and shredded mozzarella cheese evenly over the vegetables.
Pour the egg mixture evenly over all the layers in the baking dish.
Cover the baking dish tightly with aluminum foil.
To freeze: Place the covered casserole in the freezer for up to 3 months.
To bake from frozen: Preheat oven to 350°F (175°C). Remove foil and cover casserole with fresh foil. Bake covered for 45 minutes, then remove foil and bake an additional 15 minutes or until the eggs are set and the top is golden brown.
To bake from thawed: Thaw casserole in the refrigerator overnight. Preheat oven to 350°F (175°C). Bake uncovered for 40-45 minutes or until eggs are fully set and the top is lightly browned.
Let the casserole rest for 5 minutes before slicing and serving.