Gather all ingredients.

Preheat the oven to 350 degrees F (175 degrees C). Grease a deep-sided 8x8-inch square pan; add hash brown potatoes and arrange in an even layer.

Place sausage and onion in a large skillet over medium heat. Cook and stir until sausage is crumbly and browned, about 10 minutes; drain.

Meanwhile, whisk eggs, milk, onion powder, garlic powder, salt, and pepper together in a large bowl until well combined.

Pour over potatoes and sprinkle with 1/2 of the Cheddar cheese.

Add sausage mixture on top and sprinkle with remaining Cheddar. Cover the dish with aluminum foil.

Bake in the preheated oven for 1 hour. Remove the foil; return casserole to the oven and continue baking until a knife inserted into the center comes out clean, about 10 more minutes.

Let stand for 5 minutes before serving.