In a large skillet over medium heat, add the pork breakfast sausage and diced yellow onion. Cook for about 6-8 minutes, breaking up the sausage with a spoon until browned and fragrant.

While the sausage mixture cools, lay the cubed sourdough bread evenly across the bottom of a greased 9x13 inch casserole dish.

Evenly distribute the cooked sausage and onion mixture over the layered bread cubes.

In a large mixing bowl, whisk together the eggs, whole milk, heavy cream, Dijon mustard, salt, black pepper, and ground nutmeg until smooth.

Pour the egg custard mixture generously over the bread and sausage in the casserole dish.

Sprinkle the Gruyère cheese and Parmesan cheese evenly over the top.

Cover the casserole and refrigerate for at least 1 hour, ideally overnight.

Preheat your oven to 350°F (175°C) and bake the casserole uncovered for 35-45 minutes.

Allow to cool for about 10 minutes before slicing and serving warm.