In a well-seasoned 8 or 9-inch cast iron skillet, cook the bacon over medium heat until crispy and fat has rendered. Transfer bacon to a paper towel-lined plate to drain. Add the onion to the skillet and reduce heat to medium low. Cook until onion is soft and translucent, about 4 minutes.

In a large bowl, whisk together the eggs, milk, salt and pepper. Stir in the hash browns, cheese, bacon and onion.

Make sure the cast iron skillet is well greased with bacon grease, and grease with non-stick cooking spray if needed. Dump the hash brown mixture into the skillet and spread it evenly in the pan. Bake until edges are crispy, about 30-35 minutes. Serve immediately.