Preheat oven to 375°F (190°C) and grease a 9x13 inch baking dish.
In a large skillet over medium-high heat, brown the ground beef with diced onions until fully cooked. Drain excess fat.
Spread the thawed hash browns evenly across the bottom of the prepared baking dish.
Layer the cooked beef and onion mixture over the hash browns.
In a medium bowl, whisk together eggs, milk, salt, pepper, and garlic powder until well combined.
Pour the egg mixture evenly over the beef and hash brown layers.
Sprinkle the shredded cheddar cheese uniformly over the top.
Bake for 40 to 45 minutes until the eggs are set and the cheese is golden brown.
Allow the casserole to rest for 5 to 10 minutes before slicing and serving.