In a large skillet over medium heat, add the pork breakfast sausage and chopped yellow onion. Cook for about 8-10 minutes until browned and onions are translucent.

While the sausage is cooking, whisk together eggs, milk, cream, mustard, salt, pepper, and nutmeg in a mixing bowl until smooth.

In a 9x13-inch baking dish, evenly distribute cubed sourdough bread. Sprinkle the cooked sausage mixture and both cheeses over the bread.

Pour the egg custard mixture evenly over the layered ingredients, ensuring the bread is soaked thoroughly.

Cover with plastic wrap or foil and chill in the refrigerator for at least 1 hour, preferably overnight.

Preheat oven to 350°F (175°C). Bake uncovered for 45-50 minutes until the top is golden brown.

Let the casserole rest for about 10 minutes before slicing. Serve warm.