Preheat the Oven: Start by preheating your oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish.
Cook the Sausage: In a large skillet, cook the crumbled sausage over medium heat until browned. Drain excess fat and set aside.
Mix the Eggs: In a large bowl, whisk together eggs, milk, sour cream, garlic powder, onion powder, salt, and black pepper until well combined.
Assemble the Base: Spread the thawed hash browns evenly across the prepared baking dish. Sprinkle the cooked sausage over the top.
Add the Cheese: Layer shredded cheddar and mozzarella cheese over the sausage and hash browns.
Pour the Egg Mixture: Carefully pour the egg mixture over the ingredients in the baking dish, ensuring it spreads evenly.
Bake: Cover the dish with aluminum foil and bake for 30 minutes. Remove the foil and bake for an additional 20-25 minutes, or until the casserole is set and golden on top.