Cook sausage: In a skillet, cook sausage (and onion if using) until browned. Drain excess grease.
Prepare baking dish: Grease a 9×13-inch baking dish. Spread thawed hash browns evenly at the bottom.
Layer sausage & cheese: Sprinkle cooked sausage over the hash browns, then top with 1 ½ cups of shredded cheese.
Make egg mixture: In a bowl, whisk together eggs, milk, garlic powder, salt, pepper, and paprika.
Assemble: Pour the egg mixture evenly over the sausage & hash browns. Sprinkle remaining cheese on top.
Bake: Place in a preheated oven at 350°F (175°C) for 40–45 minutes, or until eggs are set and the top is golden brown.
Cool & serve: Let rest for 5 minutes before slicing.
