Preheat the oven to 350°F.
In a large skillet over medium heat, cook the breakfast sausage until browned and fully cooked, breaking it apart as it cooks. Drain any excess grease and set aside.
In a large mixing bowl, combine the thawed hashbrowns, cooked sausage, cheddar cheese, mozzarella cheese, garlic powder, onion powder, salt, and pepper. Mix well to combine.
In a separate bowl, whisk together the eggs and milk until well blended. Pour the egg mixture over the hashbrown mixture and stir until everything is evenly coated.
Transfer the mixture to a greased 9x13-inch baking dish, spreading it out evenly. If desired, sprinkle chopped green onions on top for added flavor.
Cover the baking dish with aluminum foil and bake in the preheated oven for 30 minutes. Remove the foil and bake for an additional 15-20 minutes, or until the casserole is set and the top is golden brown.
Let the casserole cool for about 10 minutes before slicing and serving.