In a large mixing bowl, whisk together 12 large eggs, 2 cups of 2% milk, salt, and pepper until smooth.

Grease the slow cooker insert. Layer one-third of the 30 oz. hash brown potatoes, followed by one-third of the 2 cups of ham, a third of the chopped onion, and a third of the 4 cups of cheese. Repeat this layering twice more, finishing with cheese.

Pour the egg mixture over the layered ingredients and press down gently to ensure the egg seeps throughout.

Cover the slow cooker with plastic wrap and refrigerate overnight.

On the serving day, place the insert in the slow cooker and cook on low for 4 to 5 hours until edges are set and lightly browned.

Let the casserole rest uncovered for about 30 minutes before serving warm, cut into squares.