Prepare the oven and baking dish. Preheat the oven to 350°F (175°C). Thoroughly grease an 8-inch square baking pan.
Arrange the potato layer. Spread the thawed shredded hash brown potatoes evenly across the bottom of the prepared baking dish to form the casserole base.
Cook the sausage and onion. Place the pork sausage and diced onion in a large skillet over medium heat. Cook for approximately 10 minutes, breaking the sausage into crumbles as it cooks, until the meat is browned and the onion has softened. Drain away any excess fat.
Combine the egg mixture. Add the eggs, milk, onion powder, garlic powder, salt, and ground black pepper to a large mixing bowl. Whisk thoroughly until the mixture is uniform and well blended.
Layer the casserole. Pour the prepared egg mixture evenly over the hash browns. Scatter half of the shredded Cheddar over the eggs, followed by the cooked sausage and onion mixture. Finish by distributing the remaining Cheddar cheese across the top.
Cover and bake. Cover the baking dish completely with aluminum foil and place it in the preheated oven. Bake for 1 hour.
Brown the cheese topping. Carefully remove the foil and return the casserole to the oven. Continue baking for another 10–15 minutes, or until the cheese is melted, bubbling, and golden.
Rest before serving. Remove the casserole from the oven and allow it to cool for several minutes before serving.