Whisk the eggs, milk, salt, and black pepper together in a medium bowl.
Grease a 5- to 6-quart slow cooker. Layer one-third of the hash browns, ham, onion, and cheddar in the crock, then repeat the layers twice.
Pour the egg mixture evenly over the layered ingredients. Refrigerate overnight if desired, or cook immediately.
Cover and cook on low for 4 to 5 hours, until the casserole is set and its edges begin to brown. Turn off the slow cooker, remove the insert, and let it stand uncovered for 30 minutes before serving.
Notes
Recipe facts extracted from the source Recipe JSON-LD card; the source gives a 4- to 5-hour cooking window and twelve servings.