In a large skillet, cook the breakfast sausage until browned. Drain off any excess grease.
In a separate bowl, whisk together the eggs, milk, salt, and pepper.
Grease a 6-quart slow cooker.
Spread the cooked sausage, diced potatoes, and diced onion evenly in the bottom of the slow cooker.
Pour the egg mixture over the sausage, potatoes, and onions.
Sprinkle the shredded cheddar cheese evenly over the top.
Cover and cook on low for 4-5 hours or on high for 2-3 hours, until the eggs are set and the casserole is heated through.
Let stand for a few minutes before serving.
Notes
This can be assembled the night before and refrigerated. In the morning, just place it in the slow cooker. For an even quicker breakfast, use pre-cooked diced potatoes or frozen hash browns.