Preheat oven to 375 degrees F (190 degrees C). Grease a 9x13 inch baking dish.
In a large skillet, cook the breakfast sausage and chopped onion over medium heat until the sausage is browned and the onion is softened. Drain off any excess grease.
1 pound breakfast sausage
Spread the cooked sausage and onion mixture evenly in the prepared baking dish.
Sprinkle the shredded cheddar cheese over the sausage mixture.
1 pound breakfast sausage
In a medium bowl, whisk together the eggs, milk, salt, and pepper.
1 pound breakfast sausage
Pour the egg mixture evenly over the cheese layer.
Top with the cooked and diced potatoes.
1 pound breakfast sausage
Bake for 45 minutes, or until the casserole is set and lightly golden brown.
Let stand for a few minutes before serving.
Notes
This casserole can be prepared the night before and baked in the morning. Cover and refrigerate, then bake as directed, adding a few extra minutes if needed.