Preheat your oven to 350°F (175°C) and grease an 8-inch square baking pan with cooking spray or butter.

Layer the thawed hash brown potatoes evenly across the bottom of the pan and press down gently.

Cook the sausage and onion in a skillet over medium heat until browned and tender, about 10 minutes. Drain excess fat.

Whisk together eggs, milk, onion powder, garlic powder, salt, and pepper in a large bowl until fluffy.

Pour the egg mixture over the hash browns in the baking pan, ensuring it covers every corner.

Sprinkle half of the shredded cheese over the egg mixture, followed by the cooked sausage and onion mixture, then top with remaining cheese.

Cover with aluminum foil and bake for 1 hour. Remove foil in the last 10-15 minutes to achieve a golden brown topping.

Let the casserole cool for about 5 minutes before serving to help it set.