Preheat the oven to 375°F or 190°C and grease a 9×13-inch baking dish. Pat the thawed hash browns dry with paper towels.
Cook the sausage in a skillet over medium heat, breaking it into small pieces. Drain away any excess grease once the meat is browned.
Whisk the eggs, milk, garlic powder, onion powder, salt, and black pepper in a large bowl. Stir in the sausage, hash browns, green onions, parsley, cheddar, and mozzarella.
Pour the mixture into the prepared baking dish and spread everything evenly. Press down any hash browns sitting above the egg mixture.
Bake uncovered for 40 to 45 minutes, until the center is set and the top is lightly golden. Cover loosely with foil if the cheese browns too quickly.
Let the casserole rest for 10 to 15 minutes before slicing. Serve warm or cool completely before freezing individual portions.
