Whip the eggs, milk dry mustard and salt and pepper together in a large mixing bowl. Set aside.
Spray a 3-quart crockpot or slow cooker with a cooking spray and then layer 1/2 of the bag of frozen hash browns on the bottom.
Then add layers of 1/2 of the meat, 1/2 of the onion, 1/2 of the cheese and 1/2 of the egg mixture on top of the hash browns
Repeat the layers of the hash brown potatoes, bacon, and onion
Top with the remaining cheddar cheese, and equally pour the rest of the egg mixture over the breakfast casserole. Cover with lid and cook on low for 7-8 hours