Whisk the eggs, milk, salt, and pepper in a large mixing bowl until smooth.
Stir in the frozen hash brown potatoes, diced ham, chopped onion, and 1 cup of the shredded cheddar cheese until everything is evenly coated.
Lightly grease the slow cooker insert and pour in the mixture, spreading it into an even layer.
Cover and cook on low for 6 hours until the eggs are set and the potatoes are tender.
Sprinkle the remaining 1/2 cup of cheddar cheese over the top, cover again, and cook for 15 more minutes until the cheese melts.
Turn off the slow cooker and let the casserole rest for 5 minutes before slicing and serving.