01 -
In a large skillet over medium heat, cook pork breakfast sausage and chopped onion together, stirring frequently, until the sausage is well browned and the onion is soft. Drain excess fat from the pan.
02 -
In a large mixing bowl, combine the cubed sourdough bread with the sausage mixture, shredded Gruyère cheese, and grated Parmesan cheese. Toss well to evenly distribute ingredients.
03 -
Grease a 9x13-inch baking dish using butter or non-stick spray. Transfer the bread and sausage mixture to the prepared dish, spreading it into an even layer.
04 -
In a medium bowl, whisk together eggs, whole milk, heavy cream, Dijon mustard, salt, black pepper, and ground nutmeg until smooth. Pour the egg mixture evenly over the assembled casserole.
05 -
Cover and refrigerate for at least 2 hours or overnight for best flavor. When ready to bake, preheat the oven to 350°F (175°C) and bake for 25 minutes until golden brown and the center is set.